I think Xykon is just keping the guards busy while he teleports in and does his summoning spell in the thrn room when no one is awares or suspects that he can get in.
The fact hat he has Teevo and can see all the way but not into the throne gives me reason to believe he is able to teleport in. Plus, why else would he go through all this trouble of sending in armies of undead other than the goblins can be risen as a second army... but to keep the sapphire guard busy. Cause they are a city of Pali's which means that they can turn undead... which make them almost useless except as a distraction tool