As we know vanilla forts have only wooden palisade and gate as fortifications.
Hence why any sieging army will be able to build only rams as siege equipment. No siege towers or ladders.
Though you can actually set the fortification level you want for each specific fort!
Open the the worldpkgdesc of your fort using the IWTE tool, then take a look in the "fortificationlevel' box.
It is set as 0 by default. 0 means gate/wooden pallisade level, so that siegers will be able to build only siege rams.
Now if you have a custom fort with a battlemap model with higher wall size, you can change the value in this box to one that match the wall size of your fort in battlemap.
Here is the parameters:
none = no defenses at all
0 = wooden palisade (used by town) - allows building of rams by siegers
1 = small walls (used by town large town, city) - allows building of rams, small ladders and small siege towers by siegers
2 = large walls (used by large city and all castle levels) - allows building of rams, large ladders and large siege towers by siegers
3 = huge walls (used by huge city) - allows building of rams and huge siege towers by siegers
Examples:
f you set it to 3, the siegers will be able to build huge siege towers and rams, but no ladders.
If you set it to 2, siegers will be able to build ladders, large siege towers and rams.
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SIDE NOTES:
I didnt manage to make the fort to have no fortification at all, like villages.
Tried using "unused" instead of any number (like vanilla villages) but it still allows the building of rams.
Unfortunatelly, this "fortificationlevel" box seems to be disabled for settlements, so this trick only works for forts.
Enabling this for settlements would mean a HUGE improvement for mods since some custom settlements are set as a level of walls that actually doesnt match its battlemap model. For example Erebor is a huge city, though it has no walls, only a gate. This means the siegers will be able to buld siege ladders and towers even if there is no wall to dock these engines at all. This not only messes the gameplay mechanics but also is a immersion breaker since the AI think there is walls there so they build siege towers that will not be used in battlemap. Any effort in enabling this feature for settlements would be very welcome. Maybe with the M2TWEOP project?