This has top billing on many tourist itineraries for good reason. Nowhere else does London's history come to life so vividly as in this minicity of 20 melodramatic towers stuffed to bursting with heraldry and treasure. The visitor center, opened in 2004, provides an introduction to the Tower. Systems ensure that lines are minimal, so you can put in place all those grisly torture scenes you saw in the film Elizabeth. The prize exhibit, the Crown Jewels, can be seen in glass cabinets on both sides, where moving walkways hasten progress at the busiest times.

A person was mighty privileged to be beheaded in the peace and seclusion of Tower Green instead of before the mob at Tower Hill. In fact, only seven people were ever important enough -- among them Anne Boleyn and Catherine Howard, wives two and five, respectively, of Henry VIII's six; Elizabeth I's friend Robert Devereux, the Earl of Essex; and the nine-day queen, Lady Jane Grey, aged 17. Tower Green's other function was as a corpse dumping ground when the chapel just got too full. You should know about the excellent free and fact-packed tours that depart every half hour or so from the Middle Tower. They are conducted by the 39 Yeoman Warders, better known as Beefeaters -- ex-servicemen dressed in resplendent navy-and-red (scarlet-and-gold on special occasions) Tudor outfits. Beefeaters have been guarding the tower since Henry VII appointed them in 1485.

In prime position stands the oldest part of the Tower and the most conspicuous of its buildings, the White Tower. This central keep was begun in 1078 by William the Conqueror; by the time it was completed, in 1097, it was the tallest building in London, underlining the might of those victorious Normans. Henry III (1207-72) had it whitewashed, which is where the name comes from, then used it as a barracks and as housing for his menagerie, including the first elephant ever seen in the land.

The spiral staircase -- winding clockwise to help the right-handed swordsman defend it -- is the only way up, and here you'll find the Royal Armouries, Britain's national museum of arms and armor, with about 40,000 pieces on display. One of the tower's original functions was as an arsenal, supplying armor and weapons to the kings and their armies. Henry VIII started the collection in earnest, founding a workshop at Greenwich as a kind of bespoke tailor of armor to the gentry.

Most of the interior of the White Tower has been much altered over the centuries, but the Chapel of St. John the Evangelist, downstairs from the armories, is a pure example of 11th-century Norman -- very rare, very simple, and very beautiful. The other fortifications and buildings surrounding the White Tower date from the 11th to 19th centuries. Starting from the main entrance, you can't miss the moat. Until the Duke of Wellington had it drained in 1843, this was a stinking, stagnant mush, obstinately resisting all attempts to flush it with water from the Thames. Now there's a little raven graveyard in the grassed-over channel, with touching memorials to some of the old birds.

Across the moat, the Middle Tower and the Byward Tower form the principal landward entrance, with Traitors' Gate a little farther on to the right. This is the London equivalent of Venice's Bridge of Sighs, which led to the cells in the Doge's Palace. Unlike the Venetian monument, Traitors' Gate is not architecturally beautiful but was the last walkway of daylight before condemned prisoners were doomed to darkness and death in the dungeons. During the period when the Thames was London's chief thoroughfare, this was the main entrance to the Tower.

Immediately opposite Traitors' Gate is the former Garden Tower, better known since about 1570 as the Bloody Tower. Its name comes from one of the most famous unsolved murders in history, the saga of the "little princes in the Tower." In 1483 the uncrowned boy king Edward V and his brother Richard were left here by their uncle, Richard of Gloucester, after the death of their father, Edward IV. They were never seen again, Gloucester was crowned Richard III, and in 1674 two little skeletons were found under the stairs to the White Tower. The obvious conclusions have always been drawn -- and were, in fact, even before the skeletons were discovered.

The most dazzling and most famous exhibits in the Tower are, of course, the Crown Jewels, housed in the Jewel House, Waterloo Block. You get so close to the fabled gems, you feel you could polish them (if it weren't for the wafers of bulletproof glass). Before you meet them in person, you are given a high-definition-film preview, with scenes from Elizabeth's 1953 coronation.

The Martin Tower was known as the Jewel Tower when the Crown Jewels were kept there from 1669 to 1841. These days it has a permanent exhibition entitled "Crowns & Diamonds: the Making of the Crown Jewels." See naked crown frames -- the coronation crown of George IV, George I's Imperial State Crown, Victoria's State Crown -- surrounded by 12,500 loose diamonds on permanent loan from De Beers. It's a graphic illustration of how the Royals once had to rent the stones that would adorn their headpiece on the big day.

The little chapel of St. Peter ad Vincula is the second church on the site, and conceals the remains of some 2,000 people executed at the Tower, Anne Boleyn and Catherine Howard among them. Being traitors, they were not so much buried as dumped under the flagstones, but the genteel Victorians had the courtesy to rebury their bones during renovations.

Allow at least three hours to explore, and arrive early in summer to beat the huge crowds. You can buy your ticket on the Web site, by phone, or at any Tube station. Don't forget to stroll along the battlements before you leave; from them, you get a wonderful overview of the whole Tower of London. For tickets to Ceremony of the Keys (locking of main gates, nightly 9:30-10), write well in advance (to the Queen's House, address below). Give your name, the dates you wish to attend (including alternate dates), and number of people (up to seven) in your party, and enclose a self-addressed, stamped envelope. Yeoman Warder guides leave daily from Middle Tower, subject to weather and availability, at no charge, about every 30 minutes until 3:30 in summer, 2:30 in winter. COST: £13.50; joint tickets available with Kensington Palace and Hampton Court Palace. Tube: Tower Hill.