Just played it, it's really good for a first level (My first level is just a poorly-designed grassland vanilla level), and it's a pretty decent level overall. The three parts of the level are pretty well-designed, nothing too special, but surely a solid level. I think I can give it a 7/10.
Now, for the parts that can be improved:
1. You should change the splash effect (effect-13) to brown so that it fits the color of sewer water. It looks kinda weird right now.

2. Probably it's just me, but how those brown blocks look and how they are placed made me think they're background at first. And I unironically jumped across all of them until I reached a point where it's impossible to proceed without touching those brown blocks, it was then when I found out they're actually solid blocks instead backgrounds after I carefully moved onto them.

3. This. Since the sewer water is actually lava behind a background, maybe you can fix this by making 2 different groups of sewer water backgrounds, with one group of them having higher priority.

4. The exiting pipe sound effect shouldn't play when starting at the checkpoint. A solution to this can be putting a Collectable Trigger (npc-465) at the beginning where the player spawn and making it trigger a sound effect event on death.
And, I think that's all, hopes this feedback can help!
