I don't have much to add, aside from what people already mentioned below. It's a very solid core. Needs a tiny bit of tuning in terms of visual readability (both for spawning circles and for barrels), but other than that - just needs more content, and it's a great game.
Gameplay feels deceptively fast, but the only thing that's fast is TTK. Needs a lot of on-your-feet planning. Very enjoyable.