You weren't kidding, the window scaling on this version is atrocious, I have to manually set the tab zoom on my browser to 67% to be able to see everything, including the toggle fullscreen button. Thankfully, actually toggling the fullscreen makes the game appear as it should be, at least on my 1920x1080 screen
Outside of that, it looks like upgrading the "towers" doesn't do anything during actual gameplay - their initial stats and appearance are identical no matter what stage they're at. Although so far I have only upgraded Lyra, have yet to see if it happens with Haley.
The gacha-like summoning of the towers is just a bad idea, since unless you upgrade your summoning chances, you basically have one choice of a tower, that being Lyra. Considering that a tower defense game is supposed to be a strategy, having the means to make any sort of strategy be gated away by RNG is objectively bad design. Right now the only winning strategy is to fill all slots with Lyra ASAP, the max out the summoning level and then keep summoning new towers until you get like 3-4 Haleys, so that you can replace some Lyras to beat the Boar Boss. I never even had a chance to try using Aria - since she's so rare, with the previously described strategy she comes in far too late to be of any use, since her raw damage is below the armor of Boar Boss, and you probably already have some upgraded Lyras already to deal with the normal mobs anyway. I'd personally do away with the gacha system, and replace with with a normal spend money to place towers, with each having a different base cost.
As indirectly pointed out with Aria, the overall game balance could definitely use some tweaking. The usual stuff - damage values, attack intervals, attack range etc.
The game could also use some description on what some of the buttons do, especially the ones on the right side of the tower stats panel when in gameplay. I think they're supposed to represent different tactics, but what they actually are, I don't know. I wouldn't be surprised if they didn't do anything, considering this initial release is more of a proof-of-concept rather than a fully functional demo. Adding some pop-up tooltips that appear on moving the mouse over the icon is definitely something that ought to be done.
Speaking of descriptions, some indicators for how the commander skill charges (is it based on kills or time? idk), and how long until it ends when it's activated, are also a rather obvious addition to make. Side note - since it's obvious that english is not your primary language (neither is mine tbh), Caelia's Offensive Tactics' description should be something like "Temporarily damage dealt by allies". The current description makes it sound like activating it makes it so that the allies have more damage dealt to them, and not the enemies ;).
Now don't take this comment as a condemnation of this game, because it is very, very obviously in an incredibly early state of development - again, basically a proof-of-concept. It has potential, and I'm all in on any new FMG game that isn't just a VN - not that I hate them, I just appreciate games that are something more than just a piece of interactive writing. Will look forward to any changes and additions you will make to this game.