Tower Bridge audio guide,
five stops, free.

Keep the old stone of the Tower of London at your back, face downstream, and you have the whole machine in one frame: two towers standing in the water, a pair of walkways strung between their tops, and on either side a pale blue chain swooping down to the shore. Everything on the rest of this walk is in front of you now, so give it a minute.

First, the costume. Those towers look like masonry, and they are not. Under the Cornish granite and the Portland stone is a steel skeleton, and in the bridge's own words the stone went on to protect the steelwork and to give the bridge a more pleasing appearance. The disguise was a condition of the job. The Act of Parliament of 1885 said the design should accord with the architecture of the Tower of London, the fortress behind you, and the bridge's own history says the Gothic dress answered Queen Victoria's objection to any bridge this close to it.

The man who drew it never saw it finished. Building began on the twenty-second of April 1886, and the City architect, Horace Jones, died in May 1887. His partner, the engineer John Wolfe Barry, carried it through, and Barry's architectural assistant, Mr Stevenson, swapped Jones's plainer brick front for the ornate Victorian Gothic you are looking at.

Now read the structure, because this is three bridges, not one. The chains dipping to each shore hold up suspension spans. The road between the two towers is the part that lifts, the bascules. And the walkways at the top were built as cantilevers reaching out from each tower, until cables were added in 1960 and turned them into suspension bridges too.

The colour has changed more than the shape. When the bridge opened in 1894 its ironwork was a bright chocolate brown. It went battleship grey before the Second World War, and in 1977 it was painted red, white and blue for the Queen's Silver Jubilee. The blue and white you see now comes from a repaint that began in 2008, with the old paint stripped back to bare metal.

Not everyone was charmed. In 1894 the Pall Mall Gazette called it our ugliest public work, and the architect Henry Heathcote Statham accused it of tawdriness and of falsifying the facts of its own structure. He had a point about the falsifying. He lost the argument anyway.

One last thing before you walk up to it. Find the small blue boxes on each tower, just above the sides of the road arch. They cover the ends of the quadrants inside, which would otherwise stick out when the bridge is closed. And above everything, almost sixty-three metres over the road, the crests on the tower tops are gilded with gold.

You have come in on the west side of the North Tower, and from here the visit goes up: two hundred and six steps to the top of this tower. Whether you take them or the lift, stop on a landing and look at what is actually holding the building up.

It is not the stone. From the river this tower looks like a castle keep. In here you can see what the granite hides: riveted steel girders, with yellow brick behind them. The towers were built as a skeleton of steelwork, faced outside with granite and Portland stone and backed with brickwork on the inside. At each corner of each tower stands an octagonal steel column more than a hundred and nineteen feet long.

Now the colour. The stair ironwork around you is brown, and that is not a restorer's whim. It is the bridge's original paint, a bright chocolate brown, on stairs finished in April 1894, and it is said to have been Queen Victoria's favourite colour. Outside, the bridge has since been grey, then red, white and blue, and now blue and white. In here the brown never left.

Count rivets if you like; you will not finish. There are more than thirteen million in the bridge, driven by squads of riveters as the towers rose. The steel came from William Arrol's company, which won the contract in May 1889 for eleven thousand tons of steel and twelve hundred tons of ornamental cast iron, about eight kilometres of metal in all. Arrol had started work in a cotton mill at the age of nine. By the time he took this job his firm was also rebuilding the Tay railway bridge, which had collapsed in 1879, and building the Forth Bridge. The Tay disaster is in this tower as well: the design had to withstand a wind pressure of fifty-six pounds per square foot, a rule brought in after it.

The chief engineer was John Wolfe Barry, with Henry Marc Brunel, the second son of Isambard Kingdom Brunel, as his assistant, and Edward Cruttwell as resident engineer. It took eight years, five major contractors and four hundred and thirty-two workers on site each day, some of them divers digging out the foundations inside iron caissons, the first sunk almost eight metres under the riverbed. Around thirty-one million bricks went in. Cruttwell recorded that there were only ten fatal accidents, and he meant the word only as praise.

When the two halves of the steelwork finally met over the river, Barry wrote in his diary that the last rivets slotted in perfectly. That join is still there inside the bridge. Keep climbing towards it.

You came down the South Tower and out along the Blue Line on the pavement, which names people who worked on this bridge, among them Hannah Griggs, a cook, and John Chalk, a rivet boy who started at fourteen. Now you are under the southern approach, among the machines that lifted the road for eighty-two years.

Start with the green flywheel. That colour, lined out in red and cream, was William Armstrong's own choice, taken from his family crest. Armstrong spent eleven years as a solicitor before he gave in to engineering, and by 1842 he had invented the device that makes sense of this whole place: the hydraulic accumulator. His firm, Sir W. G. Armstrong, Mitchell and Company of Elswick, signed the contract for this machinery in December 1887, for eighty-five thousand two hundred and thirty-two pounds. People called him the Magician of the North.

Here is how the magic worked. Next door, coal fires in the boilers turned water into steam. The steam drove these pumping engines; their pistons turned the flywheel and worked pumps that forced water into six accumulators, weighted cylinders that held it at seven hundred and fifty pounds per square inch. They were batteries made of water. When the bridge had to open, the stored water was released into the drive engines, which turned the cogs, which pushed on the racks at the back of the bascules, and the road went up. Look up for the accumulators: the weights are about a hundred tons each.

Nothing was trusted to a single engine. There were two pairs of engines on each pier, and the machinery was built to twice the Board of Trade's requirements, standards already tightened after the Tay Bridge disaster. It needed feeding, too. The stokers shovelled more than twenty tonnes of coal a week, and more than eighty people were needed to keep the engines running and the bridge lifting. Until the early nineteen-eighties there were cats living down here as well.

The war reached these rooms. In April 1941 a parachute mine exploded close to the bridge and seriously damaged a bascule, the towers and the engine room, and a third engine was installed during the war in case these were knocked out. It now stands in a steam museum in Norfolk.

From 1974 a new electro-hydraulic system, running on oil, was installed alongside the steam plant while the bridge kept opening, and in 1976 the old system was disconnected. By then these engines had raised the bascules more than four hundred thousand times. They have been still ever since, and they still wear Armstrong's green.