Complex sentence: Kathmandu Durbar Square in front of the old royal palace of the former Kathmandu Kingdom is one of three Durbar (royal palace) Squares in the Kathmandu Valley in Nepal, all of which are UNESCO World Heritage Sites.
Split into simpler sentences: Kathmandu Durbar Square was a plaza in front of the old royal palace of the former Kathmandu Kingdom. It was one of three Durbar (royal palace) Squares in the Kathmandu Valley in Nepal, all of which are UNESCO World Heritage Sites.