Nice graphics, although the color palette is a little biased toward primaries. The SFX are good, although there doesn't seem to be music. The economy seems reasonable, and the controls are functional and straightforward. The game seems to run efficiently (at least on my device), even when the screen is crawling with enemy units.
The one thing I felt was missing was some terrain to challenge the positioning of the player's buildings. This is an important element of tower defense games.