Birb.
In all seriousness, this has been my favorite game from the jam so far, and I genuinely think the core concept is good enough that you could expand this into a full steam game. Imagine if the player had a few more movement options and was given a little more to do during each round (e.g. melee attacks), and if you could expand your base outward in all directions onto different islands...I could even imagine you guys coming up with a cool story for the game and adding a progression system where you unlock different towers over time as you beat levels. The concept of building out and having to navigate the fortress as you go is such a great idea.
I just wish you could remove tiles if you make a mistake, and that I could put birb cages in other places besides directly below the towers. XD