One of the sites in Istanbul I really wanted to see was Rumeli Fortress. Historically speaking, this place is a big deal. Rarely does one structure have the ability to pivot history. I will have to do some more research, but the way I understand it now is that the Ottoman Turks were pushing their new Empire west, which inevitably meant battle with the declining Empire of Byzantine. Istanbul (then Constantinople) is on a strait connecting Europe and Asia.
The Ottomans wanted a strategically advantageous place to build a fortress for the eventual war. They built one super close to the city of Constantinople on the Strait of Bosporus! This is impressive for the Ottomans but stupid for the Byzantines for several reasons. First it was super close to the city, which made offensive maneuvers easier. Second, they built the fortress in 4 months! (Again, stupid of Byzantines to allow it to happen). And third, it allowed the Ottomans to control trade and military aid to the Byzantines by controlling the Strait.
I heard this incredulous story and had to see this place for myself.
Unicorns and wizard sleeves
Hammer pants and make believe
Pirate ships sailing off to sea
Will you come party with me in my castle, in my castle, in my castle, in my castle