This was an amazing game. Finding out the different viable strategies for defeating the boss was quite fun. Not to easy, but not impossible with enough patients and creativity.
The only mechanic I would change is the possible placement of towers and the bombs. Let me place them on empty green tiles too. Otherwise towers can be hard to utilize in the late game, the bombs too are useless if I have to place them to far away.
Keeping the artifacts after a death, is that an bug or a conscious decision?
Anyhow great game!