Originally the land that Westminster Cathedral is on was marshland and the Benedictine monks that belonged to Westminster Abbey made the land useful and used it for a market and fair ground. It was sold off to build a prison in the 17th century. That structure was later replaced by another prison building in 1834. The Catholic Church took ownership of the land and built Westminster Cathedral in 1903. Its design is based on the Byzantine style and is covered in mosaics. It is a Roman Catholic building and was visited by the Queen in 1995 (first time a sovereign visited a Roman Catholic liturgy).
The domes are overwhelming and have been left bare because they ran out of money. They still look amazing.
If you are in the area, stop by and its free.
American to Britain