GURPS tells us that Muirthemne didn't fall because of treachery, or incompetence on the part of its defenders--Balor simply showed up, created a huge magical inferno, and burned the city (and most of the rest of the plain) to the ground. Then he walked in, slaughtered the few survivors, poked around for any useful magical items, and left. The surviving hundred Heron Guards were those who hadn't been on the premises at the time. So it doesn't seem that the Spider Cult or any other Benedict Arnolds were responsible for Muirthemne's fall.
As a slightly more realistic missile attack, you could have the Jmen hurl rocks with their shovels lacrosse-style. It still wouldn't look particularly awe-inspiring, but I think it's more plausible and probably looks a little better than a handle shooting infinite shovel heads. Just a thought.
--SiliconDream
