A pocket change tower defense like game is definitely not what I could've predicted, I love it.
Maybe it could be nice to have some indication of what types of enemies you're likely to expect, so it opens up some strategy of when to upgrade which coins. Also not sure if it was intentional, but you can spawn new waves mid-round. It's not really game breaking (since it just makes you lose faster) but more like a funny detail to me. In some cases it could actually be used to my advantage, since my units would reach the new enemies faster if they coins were already near the right side because of the previous round. Being able to buy new coins mid-wave adds a fun extra layer of tactics and timing. At some points you get a lot of debt/enemies and have a ton of coins and pocket change to spend so it becomes kinda chaotic, but in a fun way. It's fun to throw a lot of units at a lot of enemies.
Good job on the game! The possibilities to expand upon this concept are endless, if you plan to continue with the game sometime.