Just to to know... is possible to add new maps, instead of replace them
If you redefine a terrain with own maps you want to add, you'll have to repeat the default maps. This way modders can choose to either add maps or replace them. Otherwise it would only be possible to add maps.
For example, if you have created a new farm map (CULTA terrain) named CULTA19, you created a new ruleset:
terrains:
- name: CULTA
mapBlocks:
- name: CULTA19
width: 10
length: 10
then the game will only use your map when generating a farm terrain.
To prevent this, you copypaste the terrain with all it's mapBlocks to your own ruleset and append your mapBlock to it.
(another way is to edit the default ruleset, but this is not recommended because you'll need to re-apply your changes everytime the default ruleset changes)