I love how they just turn into lasers if you level them up enough.
I also loved the game, it's pretty fun, I even played it twice (To see all of the towers' lasers) I didn't expect the towers to have completely different stats, I just assumed that the second would add to the original. I'm glad it's not like that so there's more strategy with how you combine.
The game itself is pretty solid, I didn't have any problems, except not being able to buy a tower sometimes. (Which I assume is an engine limitation rather than a bug, because I had a ton of enemies on screen when it didn't work.)
Playing the game though, could be a bit neater. I'm not gonna blame you for the lag, it's to be expected in a browser game. However, it'd be nice to see when towers are going to shoot, so you can line up your shots. And it'd also be nice to "lock" your cursor so the towers won't shoot at nothing while you buy something.
Other than those issues, still a really fun and neat game. I'm excited to see where you take this concept in the future! Good work!
P.S. It seems that the game only runs on Chrome, I couldn't get it to even load on Opera (or Opera GX) I haven't tested Firefox, but I assume it'll run there.