I usually play video games dreading that one (or sometimes more than one) "defend" mission. But in this case, I actually had a lot of fun!
My first piece of feedback is about the weapons. You added a good variety of them, but there might as well have only been one weapon: the sword. All of the other weapons had a slow fire rate and hardly did any damage compared to the sword. The sword lacked an attack animation but it did seem like I could attack almost as fast as I could press space. After I had noted the enemy spawn locations, I could simply stand slightly out of the way and spam attack my way to victory.
I also noted that bullets and enemies on average did hardly any damage to the player. However, the bullets moved far too fast for a bullet hell game as they are unreasonably fast to dodge. This meant I could continue my spawncamping spree without much of a fear of death (at least from the weaker enemies).
The difficulty curve seemed very strange as while the first few levels were over before it had all begun, level 5 and beyond seemed to have overwhelming amounts of enemies coming from all directions, making it much harder to clear. I ended up at level 6 where my skills gave way and I couldn't progress any further due to too many enemies being present at once.
Finally, I wanted to mention your tutorial. The game's mechanics were indeed explained, but I didn't note anything the tutorial said as I never had a chance to practice before the next piece of information was given to me. I would recommend a dedicated tutorial level for this, so that I could make sure that I understood each part of the game before moving on to the next one.
On the positive side, the graphics while lacking in animations were very well done and the music was very fitting. And I'm impressed by how much I got out of a game based on one of my most hated game missions.
I think that with a little more balancing and a slower pace tutorial, you might have me hooked! Nice work!