I think it is important, in order to understand Imperial warfare, to understand its inception.
In the early days of the 1600s, warfare was based around pike blocks. Early muskets reloaded slowly, so the only way to get decent firepower was to have rotating ranks, advancing alongside a heavy Tercio (in Spain) for defence. Cavalry was rendered all but useless, until Gustavus Adolphus in Sweden brought about a reniessance of the cavalry charge (dying in the proccess).
Slowly, firearms became more elaborate. Matchlock gave way to Wheel and Flintlock, streamlining the proccess of firing. Accuracy increased, pushing range of engagement further. Slowly, the great pikeblocks and their associated melee counter-infantry (such as the zwei-hander or doppelsoldner in Germany) faded away in the face of ever mounting firepower. With the advent of the bayonet, even the last purpose of the pikeman had been bestowed upon infantry of the line, ushering in a new era of warfare.

The Infantry:

As mentioned previously, Infantry had taken the principial role onto itself. Now, the vast majority of infantry deployed by the new nation-states would be "Infantry of the line," or line infantry. The use of infantry would vary on situation, but can be simplified into something like:

Line Infantry:

The main "battle" of any army would by a line of disciplined men, trained to fire by ranks. While these infantry are solid and can, if need be, charge to carry the day, they are also somewhat unspecialized. They do not shine in any particular role besides defense. Thin ranks of line infantry can deliver stunning firepower, however, and make frontal charges a risky proposal. These infantry can also be trained to form into a square. While the hollow square (a remnant of the pike blocks of old) can hold off cavalry charges, it is only truly useful on the flanks. A charge to the front of a line will be routed in short order.

Light Infantry:

Light Infantry is a blanket term for skirmishers, riflemen, or any other long range troops designed to approach the main battle line and engage from a distance before the main battle arrives. While they do not have the "staying power" of line infantry, and cannot hold the enemy on their own, they can use their mobility to stay ahead of the enemy line and cause disproportionate casualties or stage ambushes for the enemy line. Their main value is as a delaying force: a light infantry rearguard can force the enemy to deploy into line, by which time the said rearguard will have retreated and set up somewhere else. However, being small in number and fighting in loose formations makes them unable to form square and renders them vunerable to cavalry charges.

Hand Infantry

Melee infantry are still a factor as the century turns; while professional armies (the Janissary Corps in the Middle East, as well as the new standing armies in Europe) are much more uniform than their predecessors, local levies, poor peasantry, or behind-the-times local troops still resort to pike and sword. While they seem relatively useless in the face of massed musketry, a canny commander can use terrain to get them close. And expert swordsmen who get to close grips will remind even the best trained infantry they are very much still relevant.

Cavalry

Cavalry have a rich history that must be briefly looked at to understand the terminology and designations used. As the Renaissance waned, the knight armored cap-a-pie in plate slowly lost relevance due to the emergence of pike-and-shot formations throughout Europe. They were slowly replaced by mounted pistoliers who would canter up, fire, and retire from the field. These were supplemented by the dragoons, mounted infantry that would ride to battle but dismount before engaging. Then came the Thirty Years' War, and with it, the Swedes. Cavalry regained its luster as a battle winner, in Breitenfeld and Lutzen where cavalry charges shattered armies and wiped them out in entirety. As the 1700s come around, that is the context that cavalry is looked at, as the great powers begin to gather their strength. It is also one of the more prestigious arms of any military, being where minor nobility and careerists generally go to begin their lives.

Heavy Cavalry

The term "heavy" at this time is a bit misleading. It no longer refers to armor (although breastplates remain in use), but rather to the weapon and horse; a heavy pattern sword is generally straight and tip-heavy. The mount will be large and muscular, designed to smash into an enemy.
Now, the difference between the use of heavy cavalry here as opposed to an earlier medieval period is that before the advent of the flintlock, firepower had less range and a longer reload time. Cavalry now has to contend with a lot more firepower, meaning it is subject to the following:
1) Attack at an oblique angle, never straight into a line
2) Attack after the initil volley has been fired - outgoing firepower will be weaker
3) Attack lines, not columns and squares

Light Cavalry

As discussed above, the "light" designation primarily means the horses are lighter and faster. This is not shock cavalry, and their purpose is not to charge steady lines. These are, however, the men who will take your victory and seal it in blood. Artillery can rout an enemy, infantry can sieze ground, but only cavalry can destroy him in detail. Light cavalry should be used to charge wavering or routing foes and cut them to bloody pieces as they flee.
Light Cavalry are also superb raiders, looting and pillaging behind enemy lines. During Napoleon's invasion of Russia, it wasn't the cold that finished him but rather the swarms of Cossacks killing his patrols and foragers, siezing his commisary wagons and cutting his supply lines. Burn enemy farms outside of battle, or kill his artillery when engaged.

Dragoons

If heavy cavalry are the dashing picture of military might, attracting adventurers and sons of nobility, dragoons are the unwanted stepchildren. During the Thirty Years' War, the cavlary hated them and considered them a lower form of existance, classifying them as infantry. Infantry in turn resented them fr having an easier duty and riding to battle. They are also very situational in terms of use; they may be able to take and hold a chokepoint before the main battle gets there (Buford's cavalry did so on the eve of Gettysburg, in effect winning the battle before it even happened by taking and holding the hills and ridges until Union infantry made it to the battlefield). However, they are also capable of acting as light cavalry or rapidly relocating to a crisis point of battle. The main issue is their units are small, a mere sixty men that a determined attacker can simply bulldoze through where standard infantry would hold.

Ranged Cavalry

Gone are the days of Rome when mounted archers could sweep armies off the field. Likewise gone are the lumbering Tercio pikemen, easy prey for cavalry that could discharge a brace of wheellocks from beyone the waving tips of the pikes. Nevertheless, mounted gunners with pistols or carabines are decently useful - on occasion. They no longer have a cemented role. Nevertheless, as ambushers they make up for reduced stealthiness by being able to spring out of range after their first volley. This allows them to kill off generals or artillery trains - not a battle winner, to be sure, but something that nevertheless can tip the balance in your favor.

Artillery

Artillery is the proverbial glass cannon. Under ideal circumstances, they can cause horrific casualties. They can bombard from range, forcing an enemy to move. They can spew canister and grape, stopping entire assaults in their tracks. At the same time, the units manning them are small in number and they can easily be overrun if not protected.

Waterloo

I choose Waterloo not because it is a perfect battle, nor because there were no bigger, better fought, or more important battles. Waterloo is, nevertheless, the perfect illustration of what to do and how to do it (or how not to, in some cases) for a great many concepts I briefly touched on when describing the roles of various types of military body.

Preamble

Waterloo starts with the scene already set: The Prussians, though alone, have shown fight. They need to stay where they are, as they occupy one of the last roads that Wellington can take directly to them - and armies are tied to roads, if they mean to keep their ammunition wagons and cannon. Wellington himself is caught out. he has hesitate too long, and it seems the French may cut him off. There is only one crossroads that he can expect to use in order to reach Blucher's Prussians, the small crosroads at Quatre-Bras.
And the French are moving there in force.
Marshal Ney, the leader of a massive French force, arrives at Quatre-Bras and is confronted by a small force - in comparison with the army at his disposal, a minature one composed primarily of a detached group of skirmishers. Skirmishers being used here to grab a hold of crucial ground.
This is in exactly the fashion that skirmishers and dismounted dragoons can most profitably be used; they are small in number, but being fast they can race ahead and set up a roadblock that will buy valuble time for the main battle to arrive. And this is exactly what occurs: Ney hesitates, probes instead of brute forcing into the town, and by the time his courage is gathered Napoleon and Wellington have both arrived and begun setting up for the main event.

Hougemont

The next event was a bit of a sideshow, but still raises a point. Hougemont was a stone building complex anchoring the far end of Wellington's line, which he had garrisoned and reinforced. Napoleon ordered a force under his brother, Jerome, to go assault it, thinking that he'd force Wellington to commit his reserves to protecting it. It began taking too long, however, and as the resistance continued as stubbornly as before more and more French troops began being fed into the fight. The point I wanted to make here is primarily that of protecting occupied buildings or chokepoints; it can tie up disproportionate numbers of enemy troops if you do it right, buying time. There was no way for Hougemont to defeat the attackers besieging it, but it could hold out and wait for the overall battle to be won while Jerome was battering his force to bits on it's walls.

The Grand Battery

While Jerome was off smashing his face against a brick wall, Napoleon was busy setting up a Grand Battery. Being an artillery officer before his rise to power, he knew how to use his guns and he liked to concentrate them in one large artillery park to bombard an enemy. His opponent appeared content to sit on a ridge and wait, so he prepared to try and shatter it with an artillery barrage. Wellington, for his part, did his best to hide his forces below the lip of the ridge to try to muffle the effects of the barrage. This point covers two things, firstly the use of offensive artillery and defensive positioning. Trying to pull together artillery into one battery helps concentrate firepower. If the enemy refuses to move, this is a perfect move to set up an assault on that position. Secondly, terrain is something the game does not teach you very well. Nevertheless, it severely affects firepower from artillery. If you position artilley behind a hill, it cannot fire without the balls ritocheting. Likewise, hiding infantry behind a hill will shelter them until they emerge.

D'Erlon's Columns

Eventually, the bombardment sided down as a French Corps commanded by D'Erlon assembled in columns to begin an advance. Some explenation may be needed here; the game models lines (the normal way to deploy infantry) and square, a concentrated formation designed to ward off cavalry which otherwise massacres lines. In reality, however, moving troops in line was impossible. A line would falter and break when moving across terrain, and even on perfect terrain it was a difficult proposition unsuited for conscripted men pressed into service. Thus, an advance was made in a squat column (more like a fat rectangle than anything else) that would then deploy into line on contact with the enemy. D'Erlon thus began moving straight forward, a frontal assault. This was generally not the best way to attack; an offline movement to threaten a flank was better. However, both flanks were anchored by clumps of walled buildings and so D'Erlon was forced to push straight into the maw of Wellington's line, with skirmishers opening fire followed shortly by sprays of canister from the British guns. Nevertheless, the columns began cresting the ridge; they had numbers, they could absorb the losses, and may have overpowered their foe's center if given just a few more moments; parts of Wellington's line gave way and ran.
Then, through the gun smoke and dust, horses rose. A cavalry charge by multiple regiments tore through the column, which were too spread out to form square (and in contact with infantry besides). D'Erlon's attack collapsed instantly, the French retreating across the field while the cavalry pushed forward to kill as many of them as possible. Then, the recall sounded and the cavalry pulled away, returning to their original stations - that is, most of them did.
The Royal Scots Grey continued their madcap dash across the field, over to where the Grand Battery was set up. The artillery crews ran pell mell away, with the Greys in hot pursuit - right into the maw of a Polish lancer and cuirassier counter-charge that utterly swept the field clean. This illustrates multiple points; if you attack with infantry, protect them somehow from cavalry. If you attack artillery, make sure you kill all the crew (all the guns were eventually used again, as none were spiked), and if you are using cavalry be aware a fresh unit will completely massacre yours. At this point, the field was back where it had started. The French were short an infantry corps, the English had lost some infantry as well as cavalry. Then Marshal Ney decided to be an idiot.

French Cuirassier Attacks

Ney saw the effect the cavalry charge had on the enemy, and in keeping with his wild and impulsive nature he ordered them to push on into the enemy. They did as ordered, and were mercilessly mown down. The British redcoats had all the time in the world to form square, as did the Green Jackets off on the flank. This exposed them to some artillery fire, which caused casualties, but the French infantry had not yet regrouped and so the redcoats remained secure in their squares, bayonets bristling and volleys smashing into the rampaging cavalry which despite all their efforts could not break the squares; their one success occured on a side where the local British commander, a Dutch prince nicknamed "Slender Billy," decided his men would be better off in line than in square and ordered them to redeploy at the cost of almost all. This has one lesson, but one that must stick: To be truly effective, a cavalry assault must be made when infantry threathens. If the enemy can remain in square without fear of an enemy line closing, they are impervious to cavalry.

Flanking

Take a look at the following diagram: The two lines represent two regiments of infantry in lines one man thick firing at one another. Imagine a musket ball fired at one of the lines: If it has missed, if flies off into the unknown. This makes it inefficient, as you only have a given chance, say 25%, to strike an enemy with one shot.




Compare this to the next diagram:
One regiment is now off to the side of the other. It is no longer firing into the face but rather into the flank, what is called enfilading fire. A ball that misses the intended target now has a greater chance to strike another random man in that line, exponentially increasing the hits that regiment takes. In addition, the regiment that has been flanked cannot fire well at the flankers, and the men begin to panic at being so helpless. Flanking a line allows you to roll it up, with the enemy taking increased casualties and panic spreading down the entire line from there. Ambushes can be set to spring on the flank of an enemy advance, certain units can move undetected to arrive there, or ordinary infantry can take up a flanking position given that another regiment is engaging their intended target and thus pinning it in place. This also works well with artillery - a roundshot that can pass down the length of a line will cause massive casualties to that regiment.

Skirmishing

Skirmishing takes some intense management to pull off, but can be rewarding. Skirmishers, especially riflemen, have greatly increased range compared to musketeers. Instead of trying to crowd as close as possible before firing (the correct procedure for musketmen), these units should fire from extreme range. If the enemy tries to advance to contact, pull them back and halt them some distance away. When the enemy reaches their maximum range, the skirmishers will fire again and you can retire them once more. This causes some early casualties, but also breaks up the enemy advance as individual regiments try to come to gris with their tormentors and no longer advance as one coherent force.

Hopefully this was at least partially useful. If you have any questions or comments, leave them below and I will attempt to get back to you. This guide tries to mix authenticity and historical accuracy with viable gameplay. You will have to be the judges of if it succeeded. See you out on the battlefield, all!