A small problem

Umm. So i have that little platform which i want to be basically the only thing in my level. The platform stands in the sea of lava. Strange enough, there going to be lots of monsters on that platform. And some of them will come near edges, of course. Now a problem: if I shoot a monster when it's near edge, it moves forward a bit, and gets stuck on the edge. Bastard. :-\. Now, what can I do is to make the edges of the platform "Monsters cannot cross". They won't get stuck that way. But I want, for example, the cacodemons to come from the sea of lava. And they won't be able to climb up the platform and they'll have to stay in the sea (way below, so they can't even shoot at me)
What else can I do, if I can? Is there, for exmaple, some way of marking a wall "Monsters cannot cross" on one side only? Or is there any other possible solution?..
:-\
hrm.

And one more question: can I make lava that kills monsters too?

(I use zdoom)

Which source port are you using? The stuff with monsters getting stuck should not happen with MBF based ports (like MBF and PRBOOM).