"Het plein van de hoofdstad is oud."
Translation:The capital's square is old.
In English, it is perfectly acceptable to also say "capital city square" without the using the possessive.
Is there really a distinction between plein and markt ? it is not really clear, De Grote Markt in Haarlem or Antwerpen etc. Het plein in de centrum van Den Haag
I'm not a native and this is just an opinion : Markt = Market, het plein in de centrum van Den Haag = the square in the center of Den Haag... I hope I clarified something.