Unfortunately, I did not play much of the game because of a nitpick (more because of my personal setup) discussed at the end. Though, what I played of the game so far was certainly enjoyable and it had a good twist on the old tower defense formula.
The character we control (Ophi) is quite interesting mechanically but a lot of the issues I have with the gameplay flow do stem from the side stuff we have to manage with Ophi. The decision to decide whether to use the stardust to fill the blackhole and complete the level faster or use them for towers and upgrades definitely adds a layer of depth to the game. However, unless the game was balanced in such a way that punishes players that just try to put as many towers/upgrades on the map (e.g., enemies eventually outpace the towers in terms of health and speed), I think that the choice of stardust isn't as impactful anymore. The player could just focus on towers until enemies aren't a threat anymore and then feed the stardust to the blackhole. Also, the process of having to collect the stardust, much less trying to decide how the stardust should be used, as the enemies drop them is quite monotonous. Especially when more towers/upgrades are added and we want to experiment with different tower combos and placements, I can see the collection of the stardust to be a mundane part when we just want to see how the towers perform in different scenarios.
It is crazy to already be asking for quality of life features, but it would be nice if we had some way to auto-collect the stardust (maybe as an expensive option in the shop or just a permanent feature after we collect X amount of stardust in general, regardless of its usage). Then, a slider or some way to immediately specify how much we want to store in the chest (for towers and upgrades) and how much to feed to the blackhole would streamline the break period in between waves.
As others have said, the Saturn tower is pretty busted and would probably need more balancing (maybe start with that first before trying to add new towers so it would be easier to figure out the damage scaling and damage distribution for the other towers). I know the interesting part are the towers (at least for us, the players) but it might actually be easier to create different enemies first. The reason for this is because if we have all these different towers and upgrade paths with different elemental affinities but no specific enemies to exploit them with, then the strategy aspect of the tower defense game would effectively be nonexistent even if the towers were balanced.
The towers can be created on a need to counter basis -- if there is some enemy (or in some wave with increased health and speed) that cannot be handled by the current set of towers available, that is when another type of tower should be considered. Another person already mentioned the area of effect (AOE) attacks of the Saturn tower but maybe gate that tower till later in the game (or as a more expensive option) and have a simple tower that only does single target (ST) attacks early on. Gradually introduce the different mechanics of the towers with higher tier upgrades and stronger towers, but only when necessary. I would rather have a tight knit group of towers and upgrades to work with (maybe with only a couple of elemental attacks) that are well-balanced and enemies that requires certain placement and combos of towers that aren't directly countered by a single tower (like AOE towers should have lower damage than ST towers but may have special effects, like slowing enemies).
As for the reason why I didn't test out the game fully...The innocuous end bit to the description above "(Settings button does nothing at the moment)" was actually a bit disheartening. When first loading up the game at the main menu screen, everything seemed fine (besides the louder fan noise than normal). But when entering a level, the fans started revving up pretty quickly...For a simple game, I didn't think it was that intensive until I looked at the FPS counter in-game. It is certainly a weird experience to have a game be bottlenecked by the CPU, and to reach over 1000 FPS...frame caps would certainly help here. Case in point, maybe cap the frame rate at 60 FPS (or some other reasonable number) or have some way to limit it in the settings because I was pretty surprised when everything started to run really hot and immediately closed the game when running it for the first time.