Kinda nice and some unusual architecture and gameplay for HL2 in the catacombs part. Generally looks good - as far as sewers/catacombs can, anyway. Does get a little repetitive, but it's still fine considering the length. Would have wished for more city bits, like a return to the surface for instance.
In my view, there are far too many zombies, I didn't really like the constant horde fighting and additional respawning. The latter isn't a bad thing in priciple, but should have been toned down here.
I, too, had some problems finding the vent to proceed, the one in the lavatory. The gravity bits are a neat idea, though I found the jumping to be very awkward and finicky, so I ended up noclipping back up onto the bridge. No idea what's going on in that end fight - lucky it ended when it did; could have been even sooner, because it isn't clear what to do and the ghost zombies are quite annoying. Didn't really get the gist of the story... I'm just going to file it under weird.
, hopefully complete.