basically all the peeks to the “backside” (ratman’s dens?) from the test-chambers are pretty different, but they were fairly cosmetic anyway.

after the last chamber & the fire/furnace, it’s like entirely different game. Different puzzles, different … everything. Some ideas of original games’ rooms there are similar (like the giant room with many turrents), but execution is VERY different. Which, incidentally, is the place where I left off as the darn turrents killed me repeatedly. x)

And tbh, it’s been few years since I last played portal (any of them). I think I’ve played through portal 1 probably 4-5 times? So I’m not THAT much “in the know” with all the rooms/tidbits.