Besieging undead armies

[Gurthang]

I am besieging an army of undead, and since they don't need to eat I suppose it's pointless to wait for them to starve

So... what's the solution? Suppose Ermor stuffs a fortress with, say, 600 undeads. How can an average nation ever hope to storm the fortress?

I am considering to use one or more Wall Shakers here. Do they add their fortress reduction values?

All help appreciated!