Nice game, but...
it was too slow. This was partly due to my computer being 1.7GHz, which is below your recommended specs, but partly due to the game itself. I think either the code may need some optimisation, or its due to the sprites, which tend to be labour-intensive.
I played on low quality. The frame rate stayed in the region 25-30. The units were sluggish, firing delay was longer than necessary, and the delay turret rotation should have been extended to happen even when not firing, or not happen at all, to give continuity.
The upgrades system itself was quite good, but the interface wasn't very intuitive. I took me about 30 seconds to figure out how to use it, which is too long.
I played up until wave 8 in scenario mode, then died. The sluggish nature of the game made me not want to come back to it. It needs to hook you before then, a lesson I learnt with my TD game.
I noticed in reviews that this is not a serious project. If it was I would suggest spending more time fine-tuning the game to make it easier to use.