Houses of Parliament
Commonly referred to as the Palace of Westminster, the Houses of Parliament are one of the most famous and important buildings in the entirety of Britain, being the seat of the countries two parliamentary houses - The House of Lords and the House of Commons, that can be found near the Thames river in the centre of London.
It all began in 1265,when parliament was actually created, being divided into two separate houses - the Lords and the Commons. The House of Lords immediately began conducting their meetings in the Palace of Westminster, while the House of Commons didn't have a fixed location for their meetings until 1547, when they too moved to the Palace of Westminster. This cemented the Palace as the central seat of Government, and gave it the name 'The Houses of Parliament'.
However, tragedy struck in 1834, in the form of a calamitous fire that destroyed much of the buildings of the Houses of Parliament, the only surviving remnants of the disaster being the Jewel Tower, Westminster Hall, and the crypt and cloister of St. Stephens. However, architect Sir Charles Barry and his assistant Augustus Welby Pugin created a winning design for the re-building and renovation of the ruins. Their large structure stuck to the original neo Gothic design of the Palace, and worked around the remaining buildings that had survived the fire. It took over 30 years for the Houses of Parliament that we see today to be completed, and all construction was finished by 1870.
Charles Barry and his assistant designed many of the renowned aspects of the Houses of Parliament that we know and love today, including the Clock Tower (commonly known as Big Ben after it's monstrously large bell), which was originally called St. Stephens Tower, and which has a light at it's tip that is lit whenever Parliament sit at night. They also designed the Commons Chamber, which was sadly destroyed during WWII, but which was rebuilt to virtually the exact same specifications and style.
The oldest section of the Houses of Parliament is Westminster Hall, which dates all the way back to 1097.