Really nice game. The strategic placement of the buildings is really challenging and needs a good amount of clever thinking. I could imagine this having more levels with wider spaces or something like obstacles, because the idea with the buildings only working if the active field is over them is really awesome. Even that 4 x 4 field was already a challenge. Also really nice job on the mood, the song/audio in general and the beautiful graphics :)
A little node: Maybe tweek the difficulty of the waves a little more and espacially the boss at the end. While not having a problem whatsoever during almost all the waves, the difficulty of the last wave was exponentially so much higher, that it felt unfair. For context: I had the arena, a dragon and upgraded units, but only 6 of them and that was more than enought up until the boss at the end