Ok, so a lot of progress, though it has been slow because the school year has genuinely driven a train through my productivity:

• Upgrades: This is the most recent thing, but basically I added upgrades. This is handled by using a list of values instead of single values when creating a new subclass of a Tower class. I track an upgrade index and increase it to change the index and some other stuff. I made some icons and added it to the tower selected menu. I will show the coins cost of the upgrade next, because I forgot to do that. I also made new sprites for the upgrades, and a complete resprite for the cannon, which badly needed it. I also remade the tower base, which looks much better now.
• Speed setting: I added a speed setting. This was a matter of tracking a delta time multiplier, and then multiplying it with everything that uses delta time. Remember that delta time is the amount of time between frames. It lets the developer do things like move an enemy at a movement speed independent of the frame rate, because the movement is split between the delta time.
• Frostspire: I added a new Frostspire tower. This is a tower that shoots icicles at enemies and makes them slower. To do the slower part, I made an effect system that gets appended to an array in the enemy. Effects have an overlay color, and will have several attributes that are applied to the enemy if it has them. As of right now, there is just a movement speed multiplier. I want to add a final venom tower that damages enemies over time, and that should be pretty easy.
• Tank slime: I added a tank slime, which is supposed to be the final boss of the Ground Zero map. I’m not sure if I have the inspiration to make more enemies other than slimes, or the drawing skills. Essentially it’s just a much bigger slime (takes up the whole width of the path) and green, and slower, and has a lot of health. I have to take the time to balance everything though, because 2 of the basic crossbows can kill it as of right now.