No, there is no end. Fortunately there's a lot you haven't seen yet!
You can upgrade the towers. It's in the menu. There's also a combo mechanic to explore, for additional ways to kill enemies!
You can press 1-6 on your keyboard to select the various towers. You can also press S to sell and M to move towers, SPACE to pause/resume the game, and ESC to open/close the menu.
The enemies grow stronger faster on harder difficulties. If normal is too easy just go up to hard!
Thanks for playing! Congrats on getting to wave 83! At wave 100 the waves will get bigger!
Oh, and about the space issue. I tried to make space a resource that the player needs to manage in addition to the building costs. Building towers is generally more efficient than upgrading (combos blur that line a little), but eventually towers have to be placed in less and less optimal locations and eventually not at all- so you need to upgrade, combo, etc. That said, map 5 is larger than the others, if you would like more space. Of course, most of the space is unusable since the path doesn't go everywhere, but for generators it shouldn't matter.