Okay, let's look at the beginning of the level.
(http://i.imgur.com/OuaezrA.png)
If you were playing in multiplayer, having such a small ladder and a small entrance to the map would likely cause some traffic. It'd probably be good to just raise up the platform, widen the hallway, and move it a bit further away from the first room with enemies.
And, then we get to the first room with enemies.
(http://i.imgur.com/C7dAJgN.png)
This winding path is a huge no-no. It's quite annoying to have a narrow path with pits when there really isn't any reason to have it instead of a solid platform.
There's also a large, recurring problem with your textures. I'm not really sure why it be like it is, but you have sky textures that obscure the map. You can barely see it in the above screenshot, but it's much more pronounced in the one below:
(http://i.imgur.com/SOwkzTw.png)
I'll get back to that later since I'm trying to review this map in order. (Also, on a smaller note, that skybox needs work, the ball statue things only appear in one corner of the skybox.)
There are also more pits than necessary there. One general thing this map needs is to have much larger spaces; try to model it after the existing MMSP maps, it occasionally feels like a Lobstacle Course maps for the first half of the map.
Enemy placement can be a bit crowded, with you needing to give the player some room to breathe without being shot down by more than ten enemies at once.
Like this specific instance of being shot by more than ten enemies at once (nine Sniper Joes and two wall sentries):
(http://i.imgur.com/NJUPLgK.png)
Now back to the obscuring textures. This part is probably the worst instance of obscuring the map.
(http://i.imgur.com/mij4EmN.png)
It looks like the map completely cuts off. I had initially thought the map was unfinished and stopped there, until I took a leap of faith and saw:
(http://i.imgur.com/8pFe6DN.png)
Yeah, that's one of the bigger issues with the map, that spot where you can't see the area ahead of you until you make a leap of faith.
Now, after that, some of the glaring issues end for a while. The map does open up, and enemy placement, while it could still be improved, is less bloated.
The main problem now is that the map is a bit bland and same-looking.
(http://i.imgur.com/ZKugxFp.png)
A large part of it is monotexture here, and the pure black floor seems like an odd choice considering it has spikes on it.
(http://i.imgur.com/hW2dXyP.png)
This is likely the apex of blandness, where the room is just one wall, a floor, and a ceiling.
After that you get a really tight corridor; I didn't screen it, but like said earlier, you should open up that area so it flows better.
(http://i.imgur.com/oHHOil7.png)
You get to this room right before the boss, and it has several issues with it. Firstly, there's a large quantity of enemies at the bottom, making it essentially suicide to go down there if you don't carefully snipe them individually, which is annoying for the fleas that move around constantly.
Secondly, this door is so slow it's almost comical. You can't just run past the enemies, through the door, and into the boss room.
(http://i.imgur.com/0WYt33r.png)
I counted, and it takes literally five seconds for the door to reach the bottom so you can pass. And then there's another one, so it takes ten seconds overall, which is precisely why going down to the bottom is suicide. You should look at the coding Cutmanmike's MMSP maps have with the boss door and try to emulate that.
Now, as to the boss itself, it's not explicitly bad, not broken or anything, but its coding is really rudimentary. All it does is jump and shoot, then repeat. You can defeat it in a few seconds without taking a hit. I understand coding likely isn't your thing, but you should try to find someone who can code you a proper Bombman SP boss, as well as coding some original enemies for the map.
It's really nice that someone is taking initiative with making MMSP maps, but this map needs a