I find some high valor spies in the viking map esp. to be a good solution for the dilemma.

castles aren't bad but once catapult towers are integrated, it becomes pretty nasty and citadels are tough to crack with fortress almost impossible without siege.

there's also the +8 to enemy units besieged so they'll fight like hell. on expert, a total of +12 will make even peasants more or less fight to death.