This may be a known issue - siege towers probably aren't a high priority in the game's development right now. But using siege towers at Bagacum crashes the game. If they are built at the camp on the Sabis River, they crash the game when you try to use them at Bagacum. Even trying to load a save game with them in causes a crash with a message about a brigade with an invalid home. Interestingly, if they are built at Nemetacum, then they work, although they have some issues related to which slot they go to on the wall, but at least they don't crash the game. I have save games if interested.