Hey Victor! Welcome to the forums.
I guess I'll start with serious issues, since they're the most important.
First, you should always provide some outward (i.e., visible) sign that signals the player to the presence of force fields. The main scenario does this with glowy arrows, or glowy things on the walls. The convention established by the main game is that blue is for regular force fields and red for super force fields, but I see no reason why you couldn't invent your own convention as long as you stay consistent.
Second, there are many many places where the back side of walls is accessible to the player. This is bad because the back side of a wall, when touched, pulls the player's ship through the wall, doing damage the whole way. One of the worst offenders here are the part where you must blast your way through a curvy path filled with missileable blocks to a passage filled with force fields. There are also numerous passages in the top corners that are composed of single walls, with not backing. If you're not very careful with your flying, then through the walls you go.
One way around this is to give the walls an extra1 value of 0 instead of -1. However, this has the side effect that super pellets do not bounce off the walls, which is really bad for business in your level since super pellets is all you have at that point.
Another way is to make the walls not solid, and hedge them with thinner invisible walls. This way you can maintain the look and still get proper collision behavior. Note, however, that if you make the walls too thin, or the back side of one wall is too close to the front side of another, then player will be able to ram through the walls (they penetrate to the level of the opposing wall, which then pulls them through).
You should also take care that the left or right sides of a wall are not exposed. Basically, don't let the player touch any side of a wall but the out-facing side. Turning on "show contact boxes" in the editor helps identify these problems.
The other issues are less serious because they don't really affect gameplay that much, but when taken together add up to maybe one or two serious issues.
Powerups should have an extra2 flag so that they are given marker rings (unless you want to make them inconspicuous, of course). Any glowy things around them should have the same extra2 so that they are deactivated when the pickup is picked up.
There are a few places where glowy wires do not turn off when the switch is deactivated.
You don't have enough background sprites around the edges, so the player sees blur. If this were in the main playable area it would be a serious issue.
Powerups in boss areas should be behind a boss door 2 so you can't pick them up until after beating the boss. For example, when you fight the ma flicker, you can grab the green gun and run. Of course, after a while your framerate starts to tank, which causes you to die when you try to take on the narrow passages filled with enemies. (Yeah, this happened to me. :p)
Color upgrades should have the proper color, unless, again, you want them to be inconspicuous.
You should try really hard not to scale wall sprites up or down by more than around 20% (just a rough guideline); otherwise they look bad. There are three lengths of wall povided to allow you to do this.
Your walls look "chunky", again because of the scaled-up wall sprites. Having to repeat long walls over and over can be tedious, but it produces much more aesthetically pleasing levels.
IMO (this is just personal preference), a switch should open a door to an area that can't be accessed any other way; otherwise, especially in a level like yours where one part of the spiral looks much like another, players may be left wondering where that door was they opened if they've already been to the area in question.
Whew. I hope that helps a little. Again, let me say that I like your level (otherwise I wouldn't have taken the time to type all that out) and I would love to see it improved so it becomes more playable.