Single player, by 
Playing these levels I was instantly reminded of Medivo 3, and I see now they’re by the same author, so that makes sense. Once again the most obvious issue with most of these levels is there’s no reason to actually play them. It’s much faster to use copter ears to skip almost the entire map and not notice any of the details of the layout. Not every level in this pack falls into that same pattern (the Labrat level has maybe the most detailed layout), but most of them do. Three possible solutions to this problem come to mind:
1. Have walls in the air so the player can’t get too high above the ground. A few of these levels do that, but often only once, which provides only a momentary break before you’re back to soaring over everything again.
2. If you’re going to make the level’s progression almost entirely horizontal, at least have it move gradually up instead of down. Almost every level here has the start higher up than the end. The reverse would force the player to pay some attention, and would give you opportunities to design ways for the player to get upwards, such as springs, platforms, sucker tubes, pinball flippers, cheshire cats..
3. Change direction from time to time. The best levels here—Labrat, and the sewers in Colonius—do this, but the rest do not. Look at the official JJ2 single player levels for contrast… they change directions all the time. It’s fine if your overall progression is to the right, because that’s traditional for 2D video games, but there’s no reason not to have lots of moments going up, down, and left along the way. And doing this forces the player to notice your level exists and think about what to do next.
I can’t comment on enemy placement because I hardly ever interacted with any, but I did notice there were very few pickups. Collecting things is the most obvious way to provide fun/rewards to a player, so it’s not something to skimp on without good reason. The boss battles are completely fundamental and added nothing.
The graphics are fine, anyway. I wouldn’t be surprised if the background (and sometimes foreground) layers were all copied directly from the official single player levels (the start of MedivoB supports this theory), but the walls are fine too, in that they seem to tile properly even in the hardest to use tilesets like Inferno. There’s nothing innovative or elaborate about the graphic use here, but it’s all functional, and sometimes that’s all you need for a single player map.
There’s nothing wrong with trying to make a short single player level. Queen’s Castle is a great example of how to do it right—this, however, is not. Throw out most of this and instead focus on doing more of what you did in the Labrat level, where there are actual level features to interact with, a floor that isn’t just a straight line,and a layout that the player has to pay attention to.