Wow, this is a really fun game! Played it and beat Bert!
Here are my thoughts:
I start this saying I'm a great fan of tower defense genre: PvZ, Bloons Tower Defense, and those alike.
I liked the towers, the graphics, the ambience, and the 2x speed feature is always very welcome in any tower defense!
Tho, I have some critiques about the gameplay, that I hope will be useful in balancing and making the game better if you want to expand on this:
Mainly, I noticed the "interest" mechanic works not to make the player decide how to spend their money wisely, but moreso to never spend the last 1000 (because the max is 10). So, after the first 5 or 6 waves where you regain 1000, it's like it's not even there.
Also, the meta of the game is very spam-centered, as the basic towers are absolutely busted and will do the same DPS (and more) than more costly ones. Bomb towers are also absolutely useless, both in the earlygame and in the endgame, due to not having a great range or damage, and being very slow and costly. Fire towers are nice (I love flame ring tac spam in Bloons) but basic towers just outclass them in every stat. Electric towers are very strong, but do a lot of overkill damage (so reducing their overall DpS), and the presence of not very high HP enemies makes it essentially outclassed by everything that is crowd control (even the big enemies get destroyed by groups of basic towers).
I would've also liked to be able to change the targeting options during a wave for more tight situations (yes, I micromanage a lot in Bloons) as there is not much else to do after the planning phase, other than holding space.
Then, I also noticed some strange stuff going on with projectiles that seem to not have hit but actually have, but I guess that's just how the game is coded.
But overall, great game using the Tower Defense formula! I would've liked to try endless mode if I had more time! Best of luck! :D