Thanks for the feedback everyone!
I'm working on a version with a much larger outside area, so hopefully that'll be good. It's got a few more buildings and should make things more interesting if it plays well. It's almost completely different and it'll allow me to make the tanks faster (the tank speed is 65 in the version right now, instead of the normal 75, otherwise it would be far too difficult).
I'll definitely nobuild that area and make sure the sentry buster can make it through that doorway. I'm not entirely sure why the heavy got stuck where you took the screenshot, but I'll adjust it a bit so it doesn't happen again.
PMAvers: goosmurf is right, the .pop file is packed into the .bsp. Apparently crashes can occur if the .nav is packed, which is why that's packed separately.
Thanks for playing the map, everyone! I'm hoping to get the next version out sometime next month (since it's so different and I'll need to re-test all the waves internally and adjust the difficulty for the longer map)