I have to say, it's a pretty cool concept. I feel like you should probably make it more obvious what the pattern's going to look like when in game and make the map - or at least a part of it - visible before you start, to make it a bit easier to prepare for a level. It could also use a tutorial, as I didn't really understand what the text and sfx meant until I actually entered the first level. Besides that, I don't think the balance's quite right. I'm pretty sure the shotgun blast is meant to be relatively low range, but it really isn't. In most cases it's basically just a direct upgrade of your regular burst, with it taking less time, shooting out more bullets and having a more than long enough range.
Now with the negative part out of the way - I really liked basically everything else about the gameplay. I mean, the graphics obviously aren't really super polished as of right now and the audio, while pretty decent as is, should probably be worked on a bit further since it's a pretty core element of the game. But I can't really say it wasn't fun going into a room with a basically blast only build and shooting everything inside. Hope you continue working on it, since I think you've got something pretty good in here!