As both a former Conqueror and Holder of Slezkh Castle, that thing is a BITCH to attack. If Defenders hold the back side efficiently, the only thing that can bring it down is Catapults. Oh, also, from the Attacker's spawn, the tower that faces them and has an outcropping around it--you focus that tower as it is fairly weak, and once it's down, it's the best way in. It does that little phantom gimmick.
Slezkh Castle can defend against an army of 1500 with 800-1000 well armed and well trained men.