That's quite a curve for 18mm (is that usual?). My 18-50mm lens will give a slight curve at 18mm but not this much.
It's a super shot .. planet Dubrovnik!
I'd love to say it was my recently acquired jet pack, but it was just taken from the BIG tower in the city walls- not sure what the proper Croatian name for it is. I Iike the asymmetric basketball court that they've squeezed in.