Battle, by 
Rooms that are accessible only through warps are generally not a good idea, especially if they are big and you can exit them only through another warp again. People can just camp the warp targets and get plenty of kills.
If you perhaps put holes on the middle left and right the two big rooms wouldn’t be THAT campy, but there would still be a problem, as the entire map just feels cramped. There’s plenty of power ups to go with but in a small map this can result in total chaos. Also it seems the bouncer ammo boxes respawn insanely fast, which only adds up to what I said above. One warp target wasn’t even placed well, as it got me stuck in a wall.
I don’t like the eye candy. The trees in the foreground are quite distracting, it wouldn’t be that way if there wasn’t a whole lot of them. Instead you could’ve experimented with the background and added at least mountains or something.
You used the cave tiles well enough, although it seems you forgot to fix a tile bug. Other than that I don’t see any errors, Diamondus is quite an easy set to use, and it looks good. I think you should continue using this set until you get the hang of making levels.
In the future try making levels that are more open wide. Not that open but take a look at plusSeasons.j2l or Murder of Crows. They use Diamondus too and are quite good.