Pre builld map approved by devs will give too much work (i guess), but may be a limited system where you have preset option to build in your fief.
Example for village : watchtower, set of siege shield, small walls(that you can jump but protect from ranged).
For castle and town : towers, catapults, even a 2nd wall(for much gold)
Also new building that could give bonus like a smith(better gear crafting proficiency and at higher level loomed gears) garnisson(better troops recruitment proficiency or double rate) a market/farm/fishing harbor (better goods production or double rate) watchtower/milice/secret brotherhood (give info about incoming enemy like precise gear info), etc.
All this building could have several level of upgrade, need time to build and you can create only one at a time (idea from sp)