Slitherine

what is the formula for terrain defense during assault?
I know the walls give a bonus, but what exactly? Does it depend on the level? Does it depend on unbreached walls?
Do unbreached walls affect frontage?
Does terrain, e.g. mountains have an impact on terrain defense during assault. (I think so because during the assault of the self-defense the defenders had terrain defense 3).

Each level of fortification has an associated level (two in fact, there is a variant if breached). There is no way to actually know in game what will be this kind of terrain before testing it in assault. That's possible to read the data files about that though.
Exterior terrain plays no role. Defense 3 means probably you assaulted unbreached massive walls.

That would explain why assaulting unprepared solid walls with a 2 attack general can work, but assaulting massive walls under the same circumstances is akin to trying to beat down a castle gate with a spike on your helmet.

According to the data in Structures.csv and Terrain.csv, the unbreached defense bonus is +2 for palisades, +3 for city walls and solid walls, +4 for large walls and +5 for massive walls. Unbreached frontage for them is 6. For breached walls, it's +1 defense and 5 frontage for palisades, +2 defense for city walls, +1 defense and 4 frontage for solid walls, +3 defense for large walls and +4 defense for massive walls. The values assigned to the lower-level walls are rather inconsistent, but it seems that even breached, high-level walls still offer very good protection against assault. There's various other rare or faction-specific fortifications that have their own values, which I'm not going to go through.

Thanks again Soar. Most helpful.