This is the 29m tall Maiden's Tower, Baku, a place of many mysteries.
For example why is it called Maiden? There are several competing theories and many myths, most of which end up with some poor girl throwing herself from the top. More likely ideas relate to being a defensive structure that has never been captured as its real name of Qiz Qalasi can be translated as virgin tower.
Then there is the huge flying buttress to the right - why? The structure definitely doesn't need it, as the walls are 5m thick at the base. Another question is the door half way up in the picture above - where does that go?
Maybe it was unfinished and there were planned other towers that never got completed. Or maybe the buttress was a fire beacon and its original purpose was a Zoroastrian Tower of Silence.
Parts of the tower are very old, going back to the 7th - 8th Centuries BC, though much of the current form dates to the 12th Century AD.
Today its a good starting point for a tour of Baku as you get a good view of the city to plan your route.
Or alternatively if you've done the tour backwards you can see where it was you went.