Of all the Ports in all the World...

It does look like a whale!
If it is, that restraint the possible ports

 

I'm sorry, that's a leeboard, common from the late 16th century in many (smaller) sailboats in Europe and elsewhere.

 

Thanks for the correction

 

What is it for, please?

 

A leeboard is used by a sailboat, much like a centerboard, to reduce sideways movement; but located on the leeward side of the boat. The leeward side is used so that the leeboard is not lifted from the water when the boat heels, or leans under the force of the wind.

 

I searched for a port with so many steeples, as it is called "la ville aux cent clochers", maybe Rouen!

 

My vote is for Hamburg, also Hanseatic. It is bigger than Rostock and has at least one church with twin towers. There could have been another church with twin towers in the 18th century, as seen on the litho.

 

Wow! So, we've narrowed it down to Europe, then! Thanks all, for your efforts.

 

...and I appreciate your scrutiny!