Engineering wonders

[Earniel]

I was re-reading 'Tuor and his coming to Gondolin' in UT just the other day up to the part of the Gate of the Noldor. It suddenly struck me what an amazing feat of engineering and architecture it actually was, constructing a walkway next to an underground river. It may have been just an instrument for Tolkien to get Tuor where he wanted, as it is never spoken of again, but it makes you wonder what other (perhaps untold of) little miracles Elves (and Dwarves) constructed in the long ages of the world.

In the same chapter Fëanorean lamps are mentioned. These are curious little things too if you think of it. While not architectural, the skill to make something like that must have been bordering both art and science. It's telling the skill to make them did not survive on the eastern shores.

So what our your favourite (often underrated) wonders of engineering from Middle-earth?