Location: London, North bank of River Thames Date constructed: Started in 1066 Designers/Builders: William the Conqueror Function: Originally stronghold and royal residence

Today the Tower of London is a bright and welcoming tourist attraction. Situated on the north bank of the River Thames, a stone's throw from the City of London, it is a World Heritage site. The Tower is where the fabulous Crown Jewels are on display, as are many other antiquities. Visitors are greeted by "Beefeaters", in their colourful medieval uniforms. And yet the tower has a dark and sombre past.

Started in 1066, William the Conqueror built it as a protection against the "huge and brutal populace". It was just one of dozens of castles designed to overawe the conquered Saxons. Over the years the Tower was extended and altered, most notably during the 12th and 13th centuries. Despite many subsequent adaptations and restorations, the general layout of the Tower is today much the same as it was then. The interior buildings are surrounded by two concentric rings of defensive walls within a moat, which is now dry. The Traitors' Gate, through which prisoners were brought to the Tower, was originally built in the reign of Edward I.

Since its construction the Tower has served many functions. Initially it was an important stronghold and a secure royal residence. In the 13th century Henry III, threatened by aggressive barons, often had his court there and even held parliament within the Tower. The Tudors no longer used it as a residence and in military terms it was becoming obsolete. From 1100 the Tower had served as a prison - holding at various times the future Elizabeth I, the Catholic Martyr Sir Thomas More, the tragic queen Anne Boleyn and in 1952 the Kray twins, for indiscipline during army service! Despite its grim reputation not many were executed within the Tower - just seven before World War I. However, 112 were executed on nearby Tower Hill.

Over the years the Tower served as an armoury and housed a menagerie - by 1828 it contained 280 animals of 60 species. Up until the late 19th century the Royal Mint operated from the Tower and in the 14th century the Crown Jewels were lodged in the Tower, where they can be seen today.