Complex sentence: In the centre of the roundabout stands Constitution Arch (or Wellington Arch) designed by Decimus Burton as a memorial to the Duke of Wellington and originally provided a grand entrance to London, built as a northern gate to the grounds of Buckingham Palace.
Split into simpler sentences: In the centre of the roundabout stands Constitution Arch (or Wellington Arch), designed by Decimus Burton as a memorial to the Duke of Wellington and originally providing a grand entrance to London. It was built as a northern gate to the grounds of Buckingham Palace.