I like tower defense games, and having you move around the map to collect towers to place is a cool idea. This is a good foundation for a game, I'd like to see more of it.
I feel like the beginning map is a bit rectangular given you have to move around using the bridges (which is cool btw). So the later levels were more interesting, and you could make each level shorter to explore more. It would be cool to have the levels themed a bit more (e.g. parts of the farm).
Some more strategy in terms of turret placement etc. would be good (there's a bit of that later on with the two path level, which is cool).
Other than that, just some extra sounds, animation, effects etc. to polish it up would be good.
There's a good foundation here, I want to see more of it!