How to add a specific castle battlemap; Brittany:

1) Go to the folder "Medieval-Total War/Battle/Maps" containing all the battlemaps. By taking a look at the names of the castlemaps you will see the system used.

An exemple: CastleHilly02_2_1

First in the name is "Castle". Second comes the terrain(in this ex. Hilly, but could also be either "flat" or "plain").

The 02 means the second map serie with a hilly landscape; in other words a map-serie with almost identical terrain, but with different castle levels and upgrades. The most series CA has used for a given terrain-type is 5.

The _2 tell us it is a keep => _1 is a Fort,_2=Keep, _3=Castle, _4=Citadel, _5=Fortress.

The final _1 indicates "Curtain Walls" upgrade => _
0=No upgrade to the castle-level, _1=First upgrade, _2=Second upgrade


2) Open the startpos file you want to use. Find Brittany under the header
//========================================
//Region Attributes
//The special characteristics for a region
//========================================

Here the region architechture, landscape, starting culture etc. is specified. For brittany you will find the landscape to be "HILLY".
Notice the column "MAPGROUP". All regions have the
number -1. This is the number that ties a battlemap to the region. -1 simply means random, that is; the game selects a random map among the castlemaps that have the correct landscape (Brittany:Hilly) and castle-upgrade(15 levels: Fort,Fort Motte, Fort Motte with Bailey, Keep ... and so on).

3) In the startpos file: Edit the mapgroup-nr. for Brittany to for example 7 (instead of random number -1).

What you then have to do is to make a map serie for your brittany castle; 15 maps, one for each castle upgrade. It sounds harder than it is, because what you do is to make a single landscape-map and just build a larger and larger castle while saving the map with the correct name at each castle upgrade. The names of the 15 maps must now be in this form:
castlehilly07_*Castle-level*_*Upgrade-level*

The reason for using 7 instead of 6 (as CA has used the 5 first ones) is to prevent the game from selecting your brittany map for another region with mapgroup-nr set to random(-1). Apparantly, if there is no nr.6 the game will not be able to select the nr.7 randomly. Only when you instruct the game to use map-serie nr.7 for a given region in the startpos-file will it then be able to select it.

I made this mistake in my first attempt, and ended up assaulting my Jerusalem castle when I was actually taking Scotland

4) The map-files you make in the battlemap editor are saved in this folder: "Medieval-Total War/Savemaps"
You must manually move them to the "Medieval-Total War/Battle/Maps" folder for them to be available for selection in SP campaign. Each map gives two files, so one serie will give you 30 files to move.

Good luck