The massive Fortaleza de San Carlos de la Cabaña (Saint Charles of the Flock Fortress, Carretera de la Cabaña, tel. 07/862-4095, daily 10 a.m.–10 p.m., entrance CUC5 adults, children under 12 free, CUC8 for the cañonazo ceremony, guide CUC1), half a kilometer east of Castillo de los Tres Reyes del Morro , enjoys a fantastic strategic position overlooking Havana  and the harbor.
It is the largest fort in the Americas, covering 10 hectares and stretching 700 meters in length. It was built 1763–1774 following the English invasion, and cost the staggering sum of 14 million pesos—when told the cost, the king after whom it is named reached for a telescope; surely, he said, it must be large enough to see from Madrid.
Fortaleza de San Carlos de la Cabaña counted some 120 bronze cannons and mortars, plus a permanent garrison of 1,300 men . While never actually used in battle, it has been claimed that its dissuasive presence won all potential battles—a tribute to the French designer and engineer entrusted with its conception and construction. The castle has been splendidly restored.
From the north, you pass through two defensive structures before reaching the monumental baroque portal flanked by great columns with a pediment etched with the escutcheon of Kings Charles III, and a massive drawbridge over a 12-meter-deep moat, one of several moats carved from solid rock and separating individual fortress components.
Beyond the entrance gate a paved alley leads to the Plaza de Armas, centered on a grassy, tree-shaded park fronted by a 400-meter-long curtain wall. The wall— La Cortina—runs the length of the castle on its south side and formed the main gun position overlooking Havana . It is lined with cannons engraved with lyrical names such as La Hermosa (The Beautiful). The cañonazo (cannon-lighting) ceremony is held here nightly.
Opening to the plaza is a small chapel with baroque facade and charming vaulted interior. Facing it is the Museo de la Comandancia de Che, where, following the Triunfo del Revolución, Che Guevara set up his tribunals for “crimes against the security of the state.” The small museum salutes the Argentinian doctor-turned-revolutionary who played such a key part in the Cuban Revolution. His M-1 rifle, submachine gun, radio, and rucksack are among the exhibits.
A cobbled street leads west from the entrance gate to a large cannon-filled courtyard, from where steps lead down to La Divina Pastora restaurant, beside the wharf where supply ships once berthed. The adjoining Bar La Tasca (tel. 07/860-8341, daily noon–11 p.m.) overhangs the harbor and is a great place to relax with a mojito and cigar.
Facing the plaza on its north side is the Museo de la Cabaña. The museum traces the castle’s development and features uniforms and weaponry from the colonial epoch, including a representation of the cañonazo ceremony. A portal here leads into a garden—Patio de Los Jagüeyes—that once served as a cortadura, a defensive element packed with explosives that could be ignited to foil the enemy’s attempts to gain entry.
The stone block on the northeast side of the plaza has thick-walled, vaulted storage rooms (bovedas). One boveda displays 3-D models (maquetas) of each of Cuba’s castles, including a detailed model of the Cabaña. The adjoining room contains suits of armor and weaponry that spans the ancient Arab and Asian worlds and stretches back through medieval times to the Roman era.
The bovecas open to the north to cobbled Calle de la Marina, where converted barracks, armaments stores, and prisoners’ cells now contain restaurants and the Casa del Tabaco y Ron, displaying the world’s longest cigar (11 meters long).
Midway down Marina, a gate leads down to El Foso de los Laureles, a massive moat containing the execution wall where nationalist sympathizers were shot during the Wars of Independence. A cenotaph is dedicated to Juan Clemente Zenea, executed in 1871. Following the Revolution, scores of Batista supporters and “counterrevolutionaries” met a similar fate here.
On the north side of the moat, a separate fortress unit called San Julián Revellín contains examples of Soviet missiles installed during the Cuban Missile Crisis (called the October 1962 Crisis or the Caribbean Crisis by Cubans).
The rest of the fortress grounds is still used as a military base and is off-limits. It includes the domed Observatorio Nacional (National Observatory).
A ferry (10 centavos) runs to Casablanca every 20 minutes or so from the Muelle Luz (Av. del Puerto y Calle Santa Clara) in Habana Vieja . You can walk uphill from Casablanca to an easterly entrance gate to the Foso de los Laureles. This gate closes at dusk, so don’t take this route if you plan on seeing the cañonazo.
The ceremonía del cañonazo (cannon-firing ceremony, tel. 07/862-0671; CUC4, but CUC6) is held nightly at 8:30 p.m. at the Fortaleza de San Carlos de la Cabaña, where troops dressed in 18th-century military garb and led by fife and drum light the fuse of a cannon to announce the closing of the city gates, maintaining a tradition going back centuries.
You are greeted at the castle gates by soldiers in traditional uniform, and the place is lit by flaming lanterns. About 8:50 p.m. a cry rings out, announcing the procession of soldiers marching across the plaza bearing muskets, while a torchbearer lights flaming barrels. The soldiers ascend to the cannon, which they prepare with ramrod and live charge. When the soldier puts the torch to the cannon, you have about three seconds before the thunderous boom. Your heart skips a beat. But it’s all over in a millisecond, and the troops march away.
Be sure to get there no later than 8 p.m. if you wish to secure a place close to the cannon. Hotel tour desks offer excursions.