This summer I have been playing aoe2 a lot again and rediscovering my love for it and this game reminds me of it. So I feel like this game was made just for me :). I would've never thought of stacking the sprites to create an isometric art-style in the way you did, I love it. You get 5 stars for sprites use.
The combat feels really good. Hitting multiple enemies with one swing is great and seeing the ridiculous effect of a level 8 bow is too satisfying. The arc of the arrows looks really cool aswell. The simple white flash when hitting an enemy reminds me of 2D zelda's and is just perfect. You made a great selection for the sfx as well making the hits even more satisfying. Furthermore I think the leveling up was a great addition. Not only does it reward the player every round but it also adds to the replayability, because you can experiment with different types of builds. There are a few tips I have for the combat though. The sword swinging dudes attack too abruptly. The player should be able to predict the enemy's actions so they can react and master the mechanics. Here it felt more like I was waiting for them to swing so I could punish them. The combat devolves into waiting and attacking which I don't like in games. Another thing is that it seems pointless to level up your sword beyond level 3 as you can insta-kill anyone from that point on. Adding more range to the sword might be good, but that can make the player too strong again.
I also have a tiny tiny nitpick about the 'getting hit' animation for the player. The player flashes white when being hit just like the enemies, but the hit would seem a bit more severe if and hard hitting if it was another color like red, the classic panic color.
there also was a glitch were enemies couldn't be bothered to steal the relics. As though the newer waves swore to avenge their fallen warriors and were blinded by rage. But you were already awere of that.
There were a lot of really nice little touches like the spirits leaving the bodies of the fallen enemies. And I liked your voice over and sense of humor. The story is short and reinforces what the player has to do as it should be.
There wasn't really a lot of enemy variaty but I know you couldn't fill the 48 hours and I bet you had many great ideas (like the WOLOLO idea). This game has an incredible amount of polish and is one of my favorite entries in the jam. Goed gedaan!!! oh sorry, my inner dutch dude jumped out again...