I think that most of the towns (many castles, too, but that's a problem for another thread), if not all of them, need to be given a good looking at once again. There's a huge disparity in map difficulty between some of them, but more importantly is something we should figure out first:
Were towns intended to be more difficult to conquer than castles, or less?
From what I've seen while playing, some towns (such as Curaw, Reyvadin), tend to have defenses which are too spread out or convoluted to defend practically. Even Dhirim's walls are poor for defense, considering the locations difficult to reach by defense, and that have no cover, that give attackers access from behind to the gatehouse and left side of the wall.
Many times it is a castle's defenses which stand better than a town's, due to being more compact or having walls that defense can reach more easily and traverse more quickly. Towns tend to devolve into a series of street fights with defenders repetitiously falling back (in-between getting spawn camped momentarily while they reset position), while castles typically stand up with defenders maintaining the walls well after most of them have been broken by catapults.
I'll also look at Dhirim, Arty, and we can compare notes sometime if you like. As much as I like the Dagger Room there, it's obviously a bit too good for defense, but I don't think it should be as easy to fight at for attackers as, say, Reyvadin. We should revisit them all, really, now that many have seen their fair share of fighting.