Very interesting concept, although I found it quite difficult to progress.
I think the UI could be better, especially with the shop, I think that the shop should be part of the icons on screen.
The enemies were odd to deal with, it would be better if they were slowly introduced and explained, instead of all coming in at once.
The turrets feel like they should do a little bit more, else be a little bit cheaper, considering that the first one disappears so soon.
An alternative solution to the turrets is to make the amount of time between waves longer and either freeze or slow down time whilst in the shop.
But, honestly, the game was really good. The magnet's concept and the variety of enemies makes the game work well and implements a nice strategy element along the interesting tower defence.