When it first started I had no idea what was going on..
But as soon as the combat started... damn!
I suddenly had an urge to defend myself from this fast little enemy (with a freaking aimbot!).
Really cool concept.
A few ideas to polish it:
A direction indicator for the closest enemy
A clear indication of whether I can or can't place or afford a new item
A different way to show my inventory and the costs
A tutorial would help with some of those elements but if you want the game to be intuitive I think all 3 would help.
Charming concept as well.