The constant retreat is a bug indeed, even in MP, coz sometimes is part of ur strategie, trying to lure the enemie to catch him after.

In STW, on bridge battles i like to use some fast Yaris cavs. As IA always wait u on the other side i deployed many archers and so on just on the edge near the bridge. Then i crossed the bridge with yaris luring some heavy units of the enemie, then back the yaris and start shooting with archs and muskts. I won many battles on this way with really poor loses.
Never tried on MTW, but i suppose it will work also.