Korgoth said:Looks rockin'.
I guess if you wanted to make it even tougher, the kobolds could trap/mine the door somehow.
Thief: "I don't recognize the script on this object."
Magic-User: "Out of way, oaf! Ah, it's written in Kobold."
Fighter: "What does it say?"
Magic-User: "Hmm. This side toward enem..."
BOOM!
That or maybe have a trap/misdirection sprung behind the party when they open the door that tends to herd them into the room/kill zone.
I'm going to move some kobolds up from the bottom to the top, replacing them with decoys. You mentioned "kill zones" and I think it looks too much like a kill zone - they might run. I want them to be tempted to charge the dais. The fact that they're silenced around the door might cause some confusion in this regard. I might even extend the silence further into the hallway so that the players just have to tell me what they do, without forming a plan...