There's a bug where a palisade built by a celtic culture in a large town means that town can't be viewed on the battle map, so you can't attack/defend it manually.
Eg. If Teamhair na Ri is captured by the vikings and developed into a large town with a palisade, it's fine.
If Leinster capture it and do the same, CTD. The bug doesn't occur if there's no palisade for any culture. And if the Albans for example build a palisade in a large town, ctd.
Let me know if anyone else has the same problem.
Edit: solved, error was due to having the wrong mod_0.pack file