I love tower defence games so this was a real blast to play. I love the idea of the person making the twoers is actually involved in the game, that added a whole new element that was really fun and different. I have a few notes but I'll get to them as I answer your questions
Does the hectic aspect seem balanced? And if not, what aspects aren't? For example, enemies being too quick/slow, etc.
The enemies were all too easy to beat. I set up one tower right on the corner and it pretty much took care of everything. and if it didn't I had a second one placed anywhere that wiped them out pretty easily. But the waves were cool and I liked the designs
Is the balance between using stardust for towers and using it to feed the black hole sufficient?
Again the problem of easy enemies kinda made it hard to tell if the balance was good or not. I never needed a lot of stardust for towers since two was enough. I will say though getting through the third level was way too long
Do you have any particular ideas for towers?
Put a monster on a platform and have it attack the other monsters like a pet
What different terrain/level topography ideas do you think would be fun and interesting?
Add a whole level of ice that was fun to skate around on and it'd make it harder to get your towers down.
Does the flow of the game feel moderately smooth, and in what ways if not?
Game flow felt smooth just kinda ran around getting everything set up
Is the UI intuitive and understandable? Any comments on adding/removing/clarifying anything to it?
The UI worked for me. I think maybe add a little area where the controls are but aside from that it worked just fine
There is currently no elemental effects/immunities implemented, so besides these, what kind of different behaviors might be interesting for enemies?
Maybe take some inspiration from plants vs zombies and add like jumpers or ones with weird physical traits that come into play as they move. Also I would say make it possible for the player to get attacked by the enemies and maybe make them lose their crystals or something so the player can actually feel like they're doing something and you make the most of having a character in a tower defence game.
Did you find any interesting strategies during your gameplay? If so, please let us know what you found, and if not, tell us how we can give you more opportunity to strategize.
I think right now the best thing to do would be just add more towers and some tougher enemies, but you already plan to do that probably