I can't through walls, huh?
I understand the idea that walls provide some reason to duck and weave more than in previous levels, but I'm not sure it's completely fair for the enemies to be able to shoot over them if I have to shoot through them. All other aspects considered, that's the on thing I'd change. If I have to shoot to get by walls, so should they.
Other than that, the only other sentiment I echo is the ability to have a "hold it down" weapon. Clicking gets pretty tiresome after the first 4 levels.
Good work.