The Original London Walks, 87 Messina Ave., P.O. Box 1708, London NW6 4LW (tel. 020/7624-3978; http://london.walks.com), the oldest established walking-tour company in London, is run by an Anglo-American journalist/actor couple, David and Mary Tucker. Their hallmarks are variety, reliability, reasonably sized groups, and -- above all -- superb guides. The renowned crime historian Donald Rumbelow, the leading authority on Jack the Ripper and the author of the classic guidebook London Walks, is a regular guide, as are several prominent actors (including classical actor Edward Petherbridge). Walks are regularly scheduled daily and cost £5 ($8) for adults, £4 ($6.40) for students and seniors; children under 15 go free. Call for schedule; no reservations needed.

Discovery Walks, 67 Chancery Lane, London WC2 (tel. 020/8530-8443; www.Jack-the-Ripper-Walk.co.uk), are themed walks led by Richard Jones, author of Frommer's Memorable Walks in London. Stepping Out (tel. 020/8881-2933; www.walklon.ndirect.co.uk) offers a series of offbeat walks led by qualified historians. Tours generally cost £5 to £12 ($8-$19).

Where London Was Born: "The City"--The best way to discover London is on foot. A jumble of ancient mews and antiques-filled alleyways, it's a city that's easy and fun to get lost in. Since many of the major sights are concentrated in specific areas of the city--historic and financial London in the City; gentlemanly London in St. James's, and so on--walking is a good way to take in a number of London sights at one time.

For more walking tours showcasing different parts of London (Royal London, Chelsea), visit the Frommer's Web site at www.frommers.com.

Start: The southern terminus of London Bridge. Tube: Northern Line to London Bridge; Northern, District, or Circle Lines to Monument.

Finish: St. Paul's Cathedral. Tube: Central Line to St. Paul's.

Time: About 3 hours, excluding interior visits.

Best Times: Weekday mornings, when the financial district is functioning, but churches aren't crowded.

Worst Times: Weekends, when the district is almost deserted.

The area known as the City--the original square mile that the Romans lived in and called "Londinium"--offers the densest concentration of historic and cultural monuments in Britain. It's also one of the financial capitals of the world--Britain's Wall Street, as it were.

Our tour begins on the southern edge of the Thames, directly west of London Bridge, one of the world's most famous bridges. Facing the Thames rises:

1. Southwark Cathedral--When it was built in the 1200s, the cathedral was an outpost of the faraway diocese of Winchester. Deconsecrated after Henry VIII's Reformation, it later housed bakeries and pigpens. Much of what you see is a result of a sorely needed 19th-century rebuilding, but its Gothic interior, with its many commemorative plaques, gives an idea of the religious power of the medieval church.

After your visit, walk across the famous:

2. London Bridge--Originally designed by Peter de Colechurch in 1176, under the patronage of Henry II, this famous bridge has been replaced several times since. Until 1729, London Bridge was the only bridge across the Thames. During the Middle Ages, the bridge was lined with shops, and houses crowded its edges. The bridge served as the showplace of severed heads--preserved in tar--of enemies of the British monarchs. (The most famous of these was Sir Thomas More, the highly vocal Lord Chancellor, beheaded in 1535.) The 1825 to 1831 version of the bridge was moved to the United States and is now located in Lake Havasu, Arizona. The current bridge was completed in 1973, and is not falling down.

After crossing the river, head east along Monument Street, the steep cobbled street that descends to the right. Detour down it a short distance to read the commemorative plaques attached to the:

3. Monument--Commemorating the Great Fire of 1666, this soaring Doric column is appropriately capped with a carved depiction of a flaming urn. The disastrous fire that it memorializes started in a bakery in nearby Pudding Lane, and raged for 4 days and nights, destroying 80% of the City. A cramped and foreboding set of stairs spirals up to the top of the monument. It's a tough climb, but the view of the cityscape, so heavily influenced after the fire by architect Sir Christopher Wren (who also designed the Monument), is worth it.

Retrace your steps toward London Bridge, but this time head northwest (right) along King William Street until you reach the Bank Underground station, within a stone's throw of some of England's most powerful financial institutions. As you reach Mansion House Place, make a left, looking right to:

4. Mansion House--This is the official residence of the Lord Mayor, built between 1739 and 1752 and designed by George Dance the Elder. A rather optimistic pediment frieze depicts London defeating Envy and ushering in Plenty. The mansion's architectural gem is the Egyptian Hall. Official banquets are staged here, but unless you get an invitation, you'll have to confine your viewing to the exterior.

From the mansion, proceed into a small passageway, St. Stephen's Row, and at the end take a left into Walbrook, the site of a brook that was paved over in medieval times. On the left is the entrance to:

5. St. Stephen Walbrook--One of Sir Christopher Wren's finest works, the splendid dome of this church served as a model for St. Paul's Cathedral. British sculptor Henry Moore carved the travertine altar under the dome in 1986.

Now walk across Walbrook and head into Bucklersbury, continuing until you reach Queen Victoria Street. Turn left on Queen Victoria Street, walk 1 block, make another left, and climb the steps outside the major entrance to Temple Court. Here on your left you can peer over the railings at:

6. The Temple of Mithras--This is London's archaeological jewel. Although it's shaped like a miniature Christian church, the temple was held sacred by the Mithraic cult, which had its origins in Iran and reached the Roman Empire prior to Christianity. The temple was built in A.D. 2nd century. Artifacts removed from the site are now in the Museum of London.

After you've looked around, walk back to the Bank Underground. Here, head east along Cornhill, which becomes Leadenhall Street. Just after the intersection with Gracechurch Street, behind Lloyd's of London, you'll come upon:

7. Leadenhall Market--Horace Jones designed the curved arcades on The City's central (and conspicuously non-financial) market in 1881. Since then, the market has housed a colorful collection of butchers, fishmongers, cheesemongers, and flowermongers, as well as pubs and restaurants.

Once you're finished buying or browsing, return to Gracechurch Street and walk north, and then right (east) on Leadenhall Street. Take the second right (south) on Lime Street, where you can admire the soaring and iconoclastically modern:

8. Lloyd's of London Building--Designed by Richard Rogers in 1986, this striking glass, steel, and concrete structure in the heart of ancient Roman London is the company's newest home. Lloyd's was founded in the 1680s as a marine insurer; today it is perhaps the most famous--and financially troubled--insurer in the world. Immense underwriting losses in the early 1990s threatened the company's survival.

Nearby are the London Metal Exchange, the London Futures and Options Exchange, and other financial institutions with worldwide clout.

Next follow Lime Street back to Leadenhall and turn left. Take the first right on Bishopsgate, and then the second right, and turn into an alleyway known as Great St. Helen's. Toward the end, you'll find the largest surviving medieval church in London:

9. St. Helen Bishopsgate--Built in the 1400s and dedicated to St. Helen, the British mother of the legendary Roman emperor Constantine, St. Helen was fashionable during the Elizabethan and Jacobean periods. The interior monuments, memorials, and grave markers are of special interest.

Exit back onto Bishopsgate and turn right (north). Two short blocks later, turn left onto Wormwood Street. Then take the first left onto Old Broad Street. Towering above you rises the modern bulk of the tallest building in Britain and the second-highest in Europe, the:

10. NatWest Tower--Richard Seifert designed the headquarters of National Westminster Bank in 1981. Its massive concrete foundations are built on top of mostly impervious clay, allowing the building to sway gently in the wind. Unfortunately, since it doesn't have an observation tower open to the public, you'll have to admire it from afar.

Continue south along Old Broad Street, noticing on your right the imposing headquarters of the:

11. London Stock Exchange--Built in the early 1960s to replace the exchange's outmoded original quarters, this institution has become much less boisterous since most of the City's financial operations went modern in 1986, transforming from face-to-face agreements between brokers to computerized deal making, with the exchange functioning as an electronic clearinghouse.

Take A Break--Continue southwest along Old Broad Street until it merges with Threadneedle Street. Cross Threadneedle Street, walk a few paces to your left, and head south along the narrow confines of Finch Lane. Cross busy Cornhill to the south side of the street. Follow it east to St. Michael's Alley to Jamaica Wine House, St. Michael's Alley, EC3 (tel. 020/7626-9496), one of Europe's oldest coffeehouses. Once a favorite hangout of London merchants and sea captains, today it dispenses ale, lager, wine, and bar snacks.

After tippling, take time to explore the medieval maze of narrow alleyways that provide shelter from roaring weekday traffic. Then head back to the major boulevard, Cornhill, just north of your refueling stop. Here, near the junction of five major streets, rises the:

12. Royal Exchange--Designed by William Tite in the early 1840s, the Royal Exchange's imposing neoclassical pediment is inset with Richard Westmacott's sculpture of Commerce. Launched by a partnership of merchants and financiers during the Elizabethan Age, the Royal Exchange was an attempt to lure European banking and trading functions from Antwerp (then the financial capital of northern Europe) to London. Frenzied trading and auctioning of raw materials continued here until 1982, when the building became the headquarters of the London International Financial Futures Exchange (LIFFE). If you want to see the excitement of live trading, get thee to the London Stock Exchange on Old Broad St., EC2.

On the opposite side of Threadneedle Street rises the massive bulk of the:

13. Bank of England--Originally established "for the Public Good and Benefit of Our People" in a charter granted in 1694 by William and Mary, this is a treasure trove of gold bullion, British banknotes, and historic archives. The only part of this massive building open to the public is the Bank of England Museum, whose entrance is on a narrow side street, Bartholomew Lane (tel. 020/7601-5793); it's open Monday through Friday from 10am to 5pm; free admission.

From the Bank of England, walk northwest along Prince's Street to Lothbury. On the northeast corner of the intersection rises yet another church by Sir Christopher Wren, this one completed in 1690:

14. St. Margaret Lothbury--Filled with statues of frolicking cupids, elaborately carved screens, and a soaring eagle near the altar, the interior is well worth a visit.

Now walk west on Lothbury, which will become Gresham Street. After passing a handful of alleyways, on your right you'll see the gardens and the grand historic facade of the:

15. Guildhall--The power base for the Lord Mayor of London since the 12th century (and continually rebuilt and enlarged since), Guildhall was the site of endless negotiations throughout the Middle Ages between the English kings (headquartered outside the City at Westminster) and the guilds, associations, and brotherhoods of the City's merchants and financiers. Today, the rituals associated with the Lord Mayor are almost as elaborate as those of the monarchy itself. The Guildhall's medieval crypt is the largest in London; Sir Christopher Wren rebuilt its east facade after the Great Fire of 1666.

Continue westward on Gresham and turn right onto Wood Street. Walk north to London Wall, and then left for about a block. On the right you'll see the modern:

16. Museum of London--Located in new quarters built in 1975, the Museum of London contains London memorabilia gathered from several earlier museums, and one of the best collections of period costumes in the world. Built on top of the western gate of the ancient Roman colony of Londinium, the museum has a strong collection of archaeological remnants unearthed during centuries of City construction, as well as dioramas portraying the Great Fire and Victorian prison cells.

Now head south on Aldersgate, which quickly becomes St. Martins-le-Grand. After you merge with Newgate Street, the enormous stately dome of one of Europe's most important churches slowly appears in front of you:

17. St. Paul's Cathedral--Sir Christopher Wren's masterpiece is known to many as the site of the wedding of Prince Charles and Princess Diana. St. Paul's also held the state funerals of Nelson, Wellington, and Churchill, and served as an inspiration to a generation of Londoners who survived the bombings of World War II. The only cathedral in England constructed with a dome, and the country's only church built in the English baroque style, St. Paul's was also the first English cathedral designed and built by a single architect.

From St. Paul's, you can catch the Underground's Central Line to your next destination.