Sigiriya is a massive rock over 200m high that has the ruins of a castle/monastery or both (depends who you ask) on top of it. Some versions of history claim it was a pleasure palace for one of the early kings of Sri Lanka. Regardless of its use, it is an amazing feat of engineering.
For me it stands out as a reminder of how ambitious and….random human beings can be. Given the technology available 1,500 years ago, who in their right mind would look at a rock like this and think, “Oh yes, I am going to build one bad ass castle on that. On second thoughts, how about I make it a pleasure palace. Oh yeah the girls will really dig me if I can take them up there on dates.”