Aside from Kakaliol who got deeper into Golgotterath than the others, most of them seem to have attacked the top most part of the fortifications, and were killed there by whatever baddies were up there. The big worm Demon apparently created a big hole in the upper walls with his body.
Again, it seems that Kellhus and Iyokus would have deduced beforehand that using demons in a topos is a bad idea. So not sure if that was a Bakker mistake, a mistake on Kellhus's part or they deemed it worth the risk ?
Also,
Iyokus keeps telling Kakaliol to fulfill his mission. What was his mission?! Just get in?