Seige Question

[NTJedi]

I've experienced the same results for some of my sieges as well. Once I had 6 siege golems, 50 fiends of darkness, 2 demonlords and about 100 other troop types... yet would get the same message. This was also a castle owned by C'Tis.
"the walls were unharmed" ?

[Taqwus]

C'tis could have a pile of undead. While mindless undead don't defend castles very well, one can accumulate large quantities and you can't starve them out.

Try bringing in Wall Shakers and Gate Cleavers to help.