a height of 135 meters is the biggest Ferris wheel in Europe, and has become the most popular paid tourist attraction in the United Kingdom. It was formally opened by the then Prime Minister, Tony Blair, on 31 December 1999, although it was not opened to the public until March 2000 because of technical problems. Since its opening, the Eye has become a major landmark and tourist attraction. The tower of London is a historic monument in central London, on the north bank of the River Thames. The Tower of London is often identified with the White Tower, the original square fortress built by William the Conqueror in 1078. However, the tower as a whole is a complex of several buildings. The tower's primary function was a fortress, a royal palace, and a prison (particularly for high status and royal prisoners). This last use has led to the phrase "sent to the Tower" (meaning "imprisoned"). It has also served as a place of execution and torture, an armoury, a treasury, a zoo, the Royal Mint, a public records office, an observatory, and since 1303, the home of the Crown Jewels of the United Kingdom. Completed in 1894, Tower Bridge was instantly considered a London icon and one of the great engineering marvels of its age. The National Gallery in Trafalgar Square was founded in 1824 and it houses a rich collection of over 2,300 paintings dating from the mid-13th century to 1900. The gallery is a non-departmental public body; its collection belongs to the public of the United Kingdom and entry to the main collection is free of charge. Trafalgar Square is a square in central London. With its position in the heart of London, it is a tourist attraction; at its centre is Nelson's Column which stands guarded by four lion statues at its base. The name commemorates the Battle of Trafalgar (1805), a British naval victory of the Napoleonic Wars.