I like how there's different room types to build from, and that you're encouraged to use the enemy rooms since those give you more powerups. The weapon is pretty fun to play with, since the tether means your positioning is pretty important. I also like how the enemy bullets leave themselves behind for a little bit, requiring you to put a little bit more thought in your movement.
I'd definitely put in some visual and audio feedback for when you hit and kill an enemy, the damage number alone isn't enough. An enemy should flash red briefly and a short audio blip should play when it takes damage. I'd also put in a small camera shake effect when you kill an enemy. Those additions alone would make the gameplay feel much more satisfying. The game would also benefit from more types of enemies. I'd add in something that explodes into bullets when it dies, so that you'd have to be more mindful of where you're shooting. Finally, I would decrease the number of enemies in the starting rooms, then scale it up. The first time I played, I was overwhelmed in suddenly dealing with 15 things shooting at me.
Great game overall, the weapon was very fun to use and I really like the level building concept. I had fun rampaging through the rooms with a gazillion damage upgrades.