During the first allegiance beta I played a Strat Clut campaign in which I eventually pushed south through West Seaxe and Mierce in order to acquire the territories of Wales required for my Long Kingdom Victory conditions. During this campaign I made extensive use of the ubiquitous rivers and streams that criss-cross Britannia. As a defensive option, it doesn't get better. The AI is happy to take the fight when you fortify across a river from them, and the resulting battle plays out much in the same way every time.
Apart from some rather interesting things going on when units get packed really tight in a "blob", which I won't pretend to understand, but which anyhow results in an extreme advantage to the human player, the main reason for the monotony is the fact that each river map presents just a single crossing.
I think having more crossing points on the maps would create more dynamic and interesting battles rather than the protracted blob fights of the single-bridge maps. I can imagine that the "battle for the bridge" concept is quite exciting and era-relevant, but I can say after having, for a couple of turns, fought about 6 of these every end-cycle that in the interest of making the game less "easy" it would be for the best to reduce the defender advantage a bit on these maps.