*spoilers*
I didn't read the sign that said "DON'T SHOOT THE BARRELS" until it was too late. But I could see that one was supposed to fall out the window and be blown up next to the manhole cover, so I gave myself a single pipebomb, blew the cover and went into the sewers. I got the yellow keycard there and was looking for a way out when I got squished by the level geometry in a low part of the tunnel. I didn't feel like starting over from my last save, so I quit there. I might finish the level tomorrow.
It's a pretty good map. The design is quite good for a first released map and the gameplay is average, I would say.