As a base idea, you have a good thing going here. I get it's in Alpha, but there are a few things I'd fix.
1. The sound. It's not that the music bad, it just really can get pretty annoying after a while. Also try to make the background music quieter, and add some in-game sound effects. Like for the monster's dying, or for the player swinging his/her sword. That way the music may not become as annoying, and you have some interactivity and response with the game.
2. Try to incorporate more ways to up the difficulty instead of just spawning more monsters. For example; the monsters take more hits to kill(EDIT: I realize now they do this afaik), they deal more damage, they move faster, they are smaller or bigger, maybe every now and then the "boss" of a certain monster shows up. After a while it can just become a little old when no new mechanics/monsters are shown as gameplay progresses.
3. The screensize of the game was a big one. I personally had to zoom my browser down to 50% so I could see the whole screen. Just keep that one in mind.
4. Going back to mechanics, try to also add a way for possible upgrades or spell progression. So maybe some store that the player can quickly pause the game to buy upgrades from, or make the spells do more damage, do less damage, have an increased fire rate. Just merely anything to keep the player's attention hooked.
TL;DR Here's what you should take into account. That the player is impatient and can lose attention easily. Sometimes they'll like an idea for only a certain amount of time, and then they'll become uninterested. With this type of game, the real goal is finding that happy medium between game-balance and keeping the player interested. Keeping the game balanced and fun, and finding ways to keep people hooked to your game. I have no complaints about graphics, they looked great to me. As did the animations and such. I hope none of this review is taken as any sort of aggression or ill-will, just offering some constructive criticism to help your game succeed.
Great work so far! Keep me posted