The hill at Fort Canning was originally home to the Princes that arrived on Singapura. In the 1930's, under British rule, work started on an underground bunker at the top of the hill. During WW2 this was used as a control room by Allied commanders much the same way as Churchill used the Cabinet War Rooms in London.
Fort Canning Park overlooks Orchard Road and is a nice place to walk around after sightseeing. Inside you can visit the Battle Box museum, Spice Garden, Raffles House and Sally Port to name a few.