This is a great game! I have a lot I want to say about it, so please bear with me. And, also, please know that I'm just expressing my thoughts in general, I know that making a game in only ten days is already a huge undertaking and nothing can be perfect! I think what you've made is amazing, which is why I want to get indepth on some things.
First, I really like the art, both for the gameplay and the screen between waves, plus the attention to detail with the stand getting really beat up when it's low on durability. I really like the circle fade in/fade out effect, (I wanted to put that into my own game and found a youtube tutorial about how to implement one, but it was slightly more complicated than I had time to delve into.) Also I'm impressed you were able to make the game navigable with just the keyboard, very handy in games like this if you don't have to constantly switch a hand from keyboard to mouse and back again.
The base gameplay is very solid and enjoyable and addictive while being very straightforward. In fact, it's so straightforward that you can't shoot diagonally, which is fine most of the time but at the start of waves most of the enemies are coming in from angles where you can't hit them, meaning you're stuck waiting for them to be in line with your shots, losing precious seconds while other aliens encroach on your stall. This could be mitigated if the player could freely roam around the map, but the invisible walls keeping them stuck further compounds the issue. Of course this is all personal preference, but my feeling is that it would be better if the player could stray as far as they wanted from their stand in order to try and take out some front runners early on, with the trade-off being that enemies from the other directions would get closer to the stand while the player has to run back toward it. It's then up to the player's skill and strategy to learn what's the optimal range that they can stray in any one direction, and the speed upgrades would also affect this more so.
Speaking of upgrades, the first one I took was a fire rate upgrade, which definitely helped. The second one was a damage increase, which didn't. The base enemies take three shots to kill without any damage upgrades, and with the first upgrade they still take three shots. Even though the damage numbers now say 4 instead of 3, it doesn't actually affect gameplay when you're shooting something with 9 health. The second damage upgrade was, therefore, a huge boost to survivability since that kills enemies in two hits instead of three. Then, the last upgrade to 5 damage per shot also does nothing significant, as it doesn't one-shot the base enemies. Perhaps the larger, rarer enemies die faster as a result of the first or third upgrade? But I can't be sure because they were too rare to properly test this out. I really do like the upgrade system and it adds a lot to replayability, in theory, but every upgrade should feel like it does something.
A bit more about the upgrade system in general! It would be extremely helpful if it could be accessed between waves (or even during waves if you wanted to get crazy.) Otherwise you wind up in a situation where at the start of a wave you have a damaged shop after barely surviving the last encounter and you know for a fact that you have no chance of winning the current one. On my last playthrough I reached wave 13 with many but not all upgrades. I knew I wasn't going to get any further than that, but the prospect of starting all the way over from wave 1 with the rest of the unlocks felt far less appealing. Another thing of note, since money is only ever earned after successfully defeating a wave, a player who has a very hard time at the start of the game is going to continue to suffer for much longer. In my first run I lost on the second wave, with the $67 that earned me the only upgrade I could afford was to recover from stuns faster, but I hadn't gotten stunned at all so I knew it wouldn't help me at all in doing better on the next run. In a worst case scenario, a player who can't get past the first wave will be soft-locked into never getting any upgrades and never being able to make progress at all.
Anyway, I've definitely rambled on way too long about this. I'm really sorry about that! Hopefully something I've said is at least slightly useful to consider. :)