Jack Straw's Castle is a relatively new building constructed in the 1960s and is, not by any stretch of the imagination, a castle. It is thought to be named after the leader of the 1381 peasants' revolt who lived on the hill where the "castle" was built.
Buildings - Misc
The former "castle" was destroyed by German bombing in 1941 but dusring its time was a favourite haunt of Charles Dickens, Bram Stoker and Karl Marx.
Links: historicengland.org.uk, en.wikipedia.org, www.hampsteadheath.net