Ok so, impossible mode! It's not as hard as you think. I will be outlaying a few things that will make this mode a breeze for you, in simple, and complex terms, so lets start!

Step #1: It is highly recommended that you start on impossible mode with previous survivors (From another city). Starting a city on impossible is suicide (I haven't been able to do it yet!). You may only pick 4 survivors unless your a politician. The 4 you need to bring is obviously yourself, a great soldier with the highest potential of defense, a scientist with the highest level of being a nerd (Joke), and the highest potential scavenger you got. You should be in charge of leading and building so dont worry about those parts. A scavenger is almost vital, scouting and scavenging along the cities. I also recommend them having Octonoo Binocs, they give a perk so you can scout further without the research

Step #2: (Optional) This ties in with step 1 with the survivors. You are going to want to train them to their maximum potential in the previous city (As long as you got one). If you want to go a step further, train all your survivors in ALL skills. Yes, I know it says your leader can train in all 5 but that is a lie. They can utilize all 5 at once, but the other survivors can as well. Keep in mind, it is more effective if it is the ACTIVE skill. Say I had a survivor trained in defense. They can still be trained in building and speed up building missions. It's faster then a regular survivor, but not as fast as it can be, due to the fact that defense would be their active skill, not building.

Step #3: This also ties into the last city. Equip your survivors with the best equipment possible. I recommend Rocket launchers or mini guns and bull dog or armored truck for the defense expert. Scavengers should have a fire axe and octonoo binoculars. A side note for scavengers, give them a armored truck if they have the driver perk. The engineer's should have any type of weapon (I dont know any that boost engineering) and a doctor's scapel. YOU! (The leader), should have any type of weapon, it can depend on the class you pick, I won't judge on that, and an item that gives 1+ to everything. You don't need to, but it is highly advised as you have all 5 skills active.

Optional Step #3 (Politian Case): As a politician, you can always bring another survivor or start with an extra one. I would advise bringing a maxed builder, this would be essentials for 1 day rebuilds, this come in extremely useful for an extra day can mean a lot of work!

Strategies: This is the strategies part. Lots of strategies can play into your survival!

#1 - Always, and I mean ALWAYS kill zombies around your perimeter. Massed zombies are always dangerous to starting forts and even you defense expert could have trouble with them and get injured. Wasting some days defending against them could mean more forming and your defenses coming apart.

#2 - Never reclaim a big building if its only attatched to 1 part of your fort, you will waste time scouting and killing zombies around it. In the time it takes for that, massed zombies can swarm in the adjacent buildings and around your entire fort if you can't handle it. Get more survivors and always scout ahead, as well as killing a bunch of zombies before doing something like this.

#3 - Never worry about survivors at the start. Try to avoid taking any in, due to some of the events and your farms sucking. Also try to avoid reclaiming buildings with survivors, it can be hard to get some, you can only get lucky with the events sometimes!

#4 - Work towards a workshop, labratory, and farms! You can build farms but it is best not to waste materials early on, as well as getting rid of some buildings, not all of them may be useless! Labratories and workshops are vital early on. Some events may call for a workshop and you will need ammunition for your people (The engineer can even have a weapon and make a very good zombie killer!). Besides the workshop, researching things in a labratory early on will help with food, zombie defense/killing, finding survivors, etc. Better walls and armor is a key thing to work towards in Impossible mode. Worry about those 2 things before first aid. If you neutralize the danger, what's the point in first aid if nothing can be done to you?

#5 - Scout little by little and try to avoid factions in a whole. The chosen ones can be extremely annoying if they are found early on. IF you got a small fort and the people in it don't want to kill zombies, bad things can certainly follow! Besides the chosen ones, many factions will always like to sabotage and send raiders on you! It doesn't matter if you do nothing to them or even if you don't look weak, they will attack/raid when they feel necessary! Even after they want to steal and try to kill your people, they won't mind trading. Be wary however, your leader will start out with only around 5-6 leadership if trained properly from the last city. They drive extremely high markups and haggling does not help too often in impossible and a leader below 8.

#6 - Hug rivers and hills like your best friend. We aren't playing on Trail, hopefully. With no zombies here and only a possibility of Gustav, this is certainly a lot more safer then hugging buildings infested with zombies!

#7 - When you got room and the time/food to acommodate more survivors, have your scavengers scavenge in areas where their is supplies and possibilities of equipment. It is important that these survivors are equipped as soon as they join the fort. Survivors early on will start with very low experience in a skill, so it's best to boost it with equipment and items. Not having anything for them is alright, but they may not work as best as they could!

That's it for the guide, there is a lot more I could go over but personal strategies can change and I wish you all the best luck with your town! I basically go over this and was very successful in a Impossible San Francisco on Epic map mode. I have yet to pass 2 years or have a survivor go. Good luck, and happy zombie killing!