First thing I really like the crystal design. Looks reall clean to me, with the animation, glow etc. I also like the overall look of the game, enemies look nice and even tho the turrets are minimalistic I liked their look.
In terms of game design, merging towers if they are close together was satisfying (bigger turret = bigger fun).Not chosing which tower you get next I liked the idea, makes it more simple for me who isn't really a tower defense player.
And for the sound design I the music and sound effects, especially the keys they felt just right.