Tower Bridge

The two Gothic towers in stone and blue steel are far more theatrical up close than in photos, built deliberately to look old beside the medieval Tower of London next door.

When a tall ship is due the road deck splits and lifts on its counterweights, and a crowd always gathers to watch the bascules rise and fall.

The high-level glass-floored walkways give a slightly unnerving view straight down onto the traffic and the river far below.

From the riverbanks and nearby bridges the whole structure frames the Thames, especially with the modern skyline of the City rising behind it.

Tower Bridge, completed in 1894 after eight years of construction, transformed the Tower of London into an island fortress crossed by a new thoroughfare, its twin Gothic towers and steel suspension cables becoming as iconic as the fortress they adorn.

The bridge's two bascule machinery rooms, one in each tower, housed the original steam engines, hydraulic accumulators and gearing that raised the two 1,000-tonne bridge leaves, a system that remained in use until 1976 when it was replaced by electro-hydraulic machinery.

The high-level walkways between the towers were originally designed to allow pedestrians to cross while the bascules were raised, but they proved unpopular and were closed for safety reasons by 1910; they reopened as an exhibition space in 1982.

It is one of London's defining landmarks and a bravura piece of Victorian engineering dressed up as a medieval gatehouse.

The Exhibition explains how the bascules and the original steam machinery worked, turning a familiar silhouette into a real machine.

Its position next to the Tower of London and the modern City of London makes it a natural centre for a walk along either bank of the Thames.