I don't recall the Battle for the Blackmont fort to have crashed the server. That was easily above 100 people. I believe it was a different issue entirely.
We didn't lag in the 300 player Blackmont/Sariant battle. We needed a server restart, and right now only two people can access the server console to do that. That needs fixing.
It's because of all the entities. All of the dwarves throwing.. snowballs.. And all the fireballs, and fire in general. And to top it off, the dragon was a bad idea.