As you probably already know, buildings have specific location requirements:

• Houses and other civic building: can be built only inside the city walls
  
• Factories: can be built both inside and outside the city walls
  
• Farms/Mines: can be built only outside the city walls

This means that the most critical sources of food are unprotected when the regional prince decide to put under siege the city, causing a "siege - shotdown", meaning that capability to fead that area of the Hansa is crippled, expecially if you produce a specific farm product (e.g. grain, or even beer due to the supply of grain needed) in only one or few cities.

Of course you want to avoid the facilieties "siege - shotdown" problem in order to maintain a stable flow of good to be sold through your convoys or distibution network, depending on your stage of the game

The solutions to the siege problem are 2:

• Have your building inside the walls
  
• Keep the regional lord happy

The latter is somewhat expensive and is not consistent because of the different types of mission that a lord can assign, for example the request to cripple the economy of another city, on which you can already have invested a lot. This is the only option to you in the early to mid game, and you shoul also keep it into account when it comes to inland cities and land routes.

The first solution is the go-to for the mid-late/late game. The farms that you build outside the walls will then be protected when a new and bigger wall is constructed, and they will keep functioning evend during a siege.

Since I'm not that kind of expert of the game, and I've not played much into the late game, I've never seen much of walls expansion, so I had to find information and do some testing.

All the credit for what follows goes to DDRJake, and what he says in the linked video:
https://www.youtube.com/watch?v=5q3cV3tuQU4&list=PLm0MDLKuRDrn_dRRKYXmck0T8PK1G1FBE&index=1

What he points out in the video from minute 53:00 is that you should build you farms (and also factories) as far as you can in the 3 roads entering your city.

The explanation for this tactic is the following:

1. AI as the mayor of the city tries to extend the wall so that all existing facilities at the start of the new wall project are protected by the wall... seems reasonable
   
2. AI, and usually also players, build the farms just outside of the existing wall; from a game design point I belive this behavior is justified in order to give a sense of realism: a farmer would always like to be near a walled town in order to seek protection from bandits and enemy pillage and also be near to the market when the croops are to be sold
   
3. The result from point 1 and 2 is that a wall upgrade will have minimum expansion potential when it is done by the AI (remember that you can only be mayor of one city, not taking into account the ones founded by you)

So if you place the facilities far from the wall in all 3 roads, the AI will build the walls accordingly, solving also any problem when it comes to housing space.
There is no drawback in doing so in case of siege since the siege will last the same amount of time (so the outside facilities closure) and most of time cities don't have any tower in defence of the gate and nearby area.

I admit that I did not see all the 167 videos of the DDRJake playlist, so what I will explain now derives from me and my testing, but probably this is also a topic covered by DDRJake.

I couldn't find any clear answear to this questions on internet: Does the wall expansion reduces the possibility of building farms? What does it happen when the wall is maxed?

I know that there is some kind of exploit to build outside of the maximum limit of the city, but I leave to you the search about that ...

What I find out testing as mayor of Lubeck and commisioning the construction of a wall is this: once the new wall is built and the old one is demolished, you can build the farms/mines inside the city wall, even in the inner city!

So given this and the max wall strategy of DDRJake, you can either max the walls yourself, or make the AI behave as you wish by placing far away your facilities in the roads. After the new large walls are constructed, you are totally free to place any building where and how it most pleases you!

That's it! The only reason I wrote this guide is to help people like me that are relative new to the game and they cannot find this kind of information clearly explained out there, I belive is not even stated in the official manual!

Regards!
NeoRS

PS: Apologies for my english, since is not my native language.