I'd reccomend making the placeable field a bit more square, as it is currently a bit difficult to place a good defense on the vertical edges.
Making the field slightly bigger every couple of waves could be a thing, if there will be enemies that need a lot more damage than usual.
Having to hold the mouse just to collect coins is a bit annoying, i actually first thought you had to click on them. Maybe make the coins just have to be hovered over?
And lastly, a fast-forward option or an indicator how much is left until the next wave could be good, instead of having to wait without knowing when it starts.
Still a very good game tho
(I accidentally made this tab the image i was trying to upload which made me start from the beggining)