There were many features I wanted to add, apart from also putting more effort in the UI and sound effects, but one that it was requested a lot by my few play testers (friends and family, of course) was the ability to have a second chance inside a level so that you could continue without needing to go back to the first wave of the level.
I thought that maybe you could have only one chance per level. Another that I thought, is that you can gain some kind of points, and pay for having that second chance. However, time wasn't enough and I didn't implement anything.
Take a look at the game and you can also suggest what additional features can be added! I'm planning on improving it.