In case you don't know yet, Ghoul is the only non-mindless low level undead. The other mindless undead count only 1/10 while defending a castle. You'll be seiged... no matter how well you play. Ghouls will be your life-saver in such situation.
You cannot reanimate under seige, but you can cast spells and "summon ally" to create troop. Something to pay attention to.
For the other players, so that nobody can blame us for helping only one side
. There is one easy formula versus the undeads: putting some cheap unroutable (Vine creatures, seasonal spirits), or expendable units (cheap archers) at the front while your priests behind actually doing the job. A 200+ undead army can be totally eliminated at no time. Also, a castle battle is deadly to the undeads no matter which sides of the wall they're at. The bottleneck stops their advance. Therefore, the priests can banish many before the undeads can even reach them.