Oleo has tested your game and found that you must have an old map.rwm, which unfortunately makes it impossible to reliably find the bug as we have different versions. Possibly it even IS the bug. You need to delete RTW\EB\Data\world\maps\base\map.rwm and start a new campaign in order to get the best stability. However you can continue the game using the technique he found out to bypass the problem in your case:

Quote Originally Posted by Oleo
I was trying some stuff out to get past the ctd. I moved a general next to the 4 casse stacks and slaugthered them with some cheats, then I ended the turn and the ctd was gone.
So what you need to do is pick a general and move him out of his settlement unless he's already outdoors. Then get the console by pressing the key below Esc. Teleport your general using "move_character <name> x,y", experiment with different values for x and y (they're map coordinates) to get him where he's needed. Then attack the Casse armies and type "auto_win attacker" before choosing autocalc. Then teleport him back where he belongs.