Jeez, I hate to say it, but I had an incredibly rough time with this game. Practically my entire playtime just felt like an absolute mess where I went from utter confusion to boring dissatisfaction. I'd like to think this has potential but it's quite difficult to see it in its current state.
As said, the initial impression was spent in confusion trying to learn the rules of the game and what the heck is going on. So many questions kept being asked:
Why is the only menu that is labeled the upgrade and exit: what do the other menu options do and why aren't they labeled? What is this tutorial trying to teach me? I like that it keeps things simple, but this is a case of being TOO simple! What is this menu that just popped up with a ballista button: is this a level up screen or something? Why do I keep getting goddamn interrupted every two seconds with this selection: I haven't even filled up the scrap gauge yet? Why when I chose a tower that time it wiped the entire map of enemies: you're not even gonna let me get the satisfaction of killing them myself? What's the damn point then? Why does only the starter ballista have this auto-target circle while the other similar ones don't? Am I even killing anything: it all just feels like a disconnected chaotic mess? Wait, why did the game just end with a goblin laughing: did I win? Did I lose? I don't think I got hit, so what happened?
And so on and so forth.
There's almost so much to point out that could be improved that it's difficult to decide what to say or where to start, but I'll try to limit it to a few key points for now:
1) Put more labeling and instructions and tooltips around to help guide players. Don't overdo it with walls of text or anything like that: less is more, but you want more than none, which is all you have currently. For example, put labels over the modes in the main menu, have the level up/upgrade screen be labeled at the top and put some instructions like 'pick a tower to add to your inventory'.
2) Have the level up/upgrade screen grey out the rest of the screen to make it more indicative that it has paused gameplay and is awaiting your selection. Also, have it appear with a bit more fanfare, like some sparkles or a literal fanfare sound effect, to make it feel like a nice event and separate the moment from regular gameplay.
3) Have more satisfying feedback for your towers damaging enemies: you know, sound effects and blood splats and stuff like that. In general the gameplay just felt so weird, like I wasn't feeling the satisfaction you're supposed to get from blasting enemies yourself. It's partly because its the towers killing them, but I think it's also because you can barely see your own bullets or hear/feel them hitting enemies, making it all feel so disconnected. I understand you're trying to make the screen less cluttered by muting your own bullets, but perhaps you could approach it differently, such as with colors/outlines to make things pop, especially since the game overall has a very dark palette.
4) Find a better way to introduce the whole aspect of only being able to aim certain towers in certain directions: the arrows do a pretty good job but it'd be nice if it could go a step further. It'd also be nice if this information was available in the tooltips in the level up/upgrade screen.
While I did have a really bad time, I don't think the concept is without merit: the twist of having to construct towers with certain powers and limitations could be cool as long as the game continues to get built up, I believe, so if you do continue, I'd like to see it in its final state.