Listen to your friends. And when you're visiting their country, go where they tell you to go. Our Parisian pals urged us to see La Rochelle, so off we went with nothing more than the promise, "You will not be disappointed."

We arrived in this seaport -- on France's west coast about 100 miles north of Bordeaux -- at dusk, just when the lights around the harbor winked on. The sidewalk cafe's hummed with music and voices, the evening promenade was in full swing, and we were smitten.

The town, with its fortresses, lighthouse and clock tower, looks like an operetta setting. It's easy to imagine flashing-eyed damsels, men flourishing swords and tattooed pirates singing their hearts out while scrambling around the 15th-century buildings facing the harbor and the Bay of Biscay. Romantic is the word.

The fortresses and towers date from the 14th century, when the port was founded. An independent, fortified city-state, the town soon grew prosperous and influential -- and because of its trading contacts, it became a haven for Protestants during the Reformation. But its freedoms came to a grisly end in the 17th century, when Cardinal Richelieu laid siege to the town. Twenty thousand citizens died of starvation, and the town lost all its customs franchises, as well as its political and religious freedom from France. By the 18th century, though, La Rochelle had bounced back, prospering once again through its trade with the New World and the West Indies.

Today, all this history is reflected in La Rochelle's bewildering yet delightful assortment of architectural styles, dating from various eras. We strolled through the large, old, central city, now reserved for pedestrians. A Gothic town hall, bristling with battlements, turrets, flags and cones, is neighbor to 15th-century half-timbered houses, now containing shops. Block after block of shadowed, arcaded streets house smart boutiques and restaurants in what were once the homes of fur traders and cloth merchants.

But the main allure of La Rochelle is the ancient inner harbor. Today it's surrounded by cafe's, shops and restaurants; the harbor is now used for pleasure and sport fishing. Two long, embracing sea walls protect it from the sea. At the ends of those walls, standing guard, loom two chunky towers dating from the late 14th century: Tour St. Nicolas and Tour de la Chaine.

We visited both towers. St. Nicolas, the oldest, grimmest and most massive, still feels like the prison it once was. We spent some time deciphering the messages scratched on the walls by the convicts, then climbed to the top for a glorious view of the red tile roofs of the town.

The Chain Tower, named for the gigantic chain the town guardians strung between the two towers when enemy ships came into sight, contains an interesting scale model of La Rochelle's glory days, before the siege of 1627. During the spring and summer months, a continuous sound and light show in the Chain Tower plays out the grim story of the siege.

Strolling around the quay from one tower to the other, a distance of perhaps a half mile, can take hours if you're a boat lover. Fishing smacks (small decked vessels with wells for storing fish), dinghies, small yachts, rowboats, catamarans and speedboats bob in the waters of the old port. While we watched a man mend his sail, a woman offered us a handbill advertising boat rides out through the harbor and into the ocean. On a balmy day, who could resist?

An hour later as we climbed ashore, dazed by the sun and the soft rocking of the motor launch, she said, "Make sure to visit our fish auction. You will enjoy it. Ask your concierge how to find it."

The serious business of selling fish in quantity goes on at the Halle a Mare'e, a block-long, sparkling-clean warehouse a few blocks from the St. Nicolas Tower. Huge bins of colorful fish line the floor, with narrow aisles between them. Men in rubber boots dash through the aisles shouting bids on the lots they want. The auctioneer shouts out the prices, which are listed on a chalkboard. It's a great, smelly, loud show.

We caught a city bus at the railroad station to visit La Pallice, the deep-water, 20th-century commercial harbor a few miles north of La Rochelle. There the fish are loaded onto ships and carried the world over. In the midst of the brisk, businesslike activity of gantry cranes, cargo vessels, boxcars and trucks, a few fishermen dangled their lines from empty piers.

One of them, an old man, told us he had lived in La Rochelle all his life.

"During World War II, La Pallice was a German submarine base. The base is still there and you can see it. Of course the Allies had to bomb it; what else could they do? But some good came of it. The port was rebuilt better than ever after the war, and now it is one of the most important in France. First I worked here as a boy, and now you can see what I do here." He held up his fishing pole. "Fish is the life here."

Beatrice Petrocchi is a free-lance writer based in Oakland, Calif.

WAYS & MEANS

GETTING THERE: Six trains depart daily from the Gare d'Austerlitz in Paris for the five-hour trip to La Rochelle. If you're driving, good roads traverse the cha~teau country of the Loire Valley.

WHAT TO DO: La Rochelle offers more than watery pleasures. A long, beautifully landscaped labyrinth of a park filled with secluded nooks, perfect for picnics, stretches from the town's olympique swimming pool to a tiny beach. (It's not much of a beach, so those in search of dunes and rougher waters catch the ferry from La Pallice to the beach resort of I~le de Re', a 15-minute ride away.)

There are a few museums, the most interesting of which is the Muse'e du Nouveau Monde, which tells the story of the port's 300-year-old link with the New World. There are also an excellent aquarium and a casino in town.

WHERE TO STAY: There are several hotels in La Rochelle. The following offer breakfasts but no other meals. Les Brises, Chemin de la Digue-Richelieu, phone 464-38-937, with glorious views and private parking; rates are approximately 240 to 470 francs ($40 to $80) double.

Le Savary, 2 Rue Alsace-Lorraine, phone 463-48-344, with elevator; rates are about 140 to 250 francs ($25 to $40). Francois I, 13 Rue Bazoges, phone 464-12-846, with rates of about 145 to 200 francs ($24 to $33).

WHERE TO EAT: Restaurant quality is high in La Rochelle and prices are fairly low. Just look for a place where the locals eat and you can't go wrong. Three examples in three price ranges: La Marmite, 14 Rue St-Jean du Perot. Delightful and expensive, but worth it -- about 120 to 300 francs ($20 to $30) per person. Les Quatre Sergents, 49 Rue St.-Jean. Good seafood in pretty surroundings near the port, with moderate prices -- 65 to 200 francs ($10 to $33). La Closerie, 20 Rue Verdie`re. Traditional cuisine at budget prices -- 85 to 100 francs ($14 to $18).

INFORMATION: The Office de Tourisme, at 10 Rue Fleuriau in La Rochelle, offers free maps and booklets, as well as organized walking tours of the city daily at 10 a.m. during the summer. Or contact the French Government Tourist Office, 610 Fifth Ave., New York, N.Y. 10020, (212) 757-1125.

Beatrice Petrocchi