I wall the entire map of off with a fence and leave 1 space open... sieges have 1 space to enter.
You can however wall off just your town and link it to 1 space along the edge of the map.. and they will always spawn there.
As long as there is only 1 path from edge of the map - to town.. They can only spawn in that 1 spot.
Once they spawn, you just lock the fence gate leading out of that box, they will just idle to death in the box.
They will only charge your town if there is a open path to the town.. this cam be used to minpulate them pretty easy.. if you look here:viewtopic.php?f=8&t=3382#p23768
and see how I have this ghoul siege used for testing. With little effort really.
This can be used to split a siege up and kill a few at a time - could probably even create a hero training box if I wanted. and level heroes to max with little risk of them dieing. Something I might try.. been thinking on it