National Museum of Ancient Art

Start at the street. Rua das Janelas Verdes runs along the slope above the river, and the name means the Street of the Green Windows. Trams and cars move slowly past pale facades. The air carries a little salt from the Tagus below. Behind one long, ochre-washed front, with its shuttered windows in rows, stands Portugal's national collection of old art. Many Lisbon residents never use the official name. They call it, simply, the Janelas Verdes.

The building began as a private home. It is a seventeenth-century aristocratic palace, linked to the Counts of Alvor, and later owned by families close to the royal court. You can still see its domestic proportions. The rooms are high, but not enormous. Staircases turn at human angles. The museum opened here in the 1880s, in the age when European capitals were turning royal and church treasures into public collections. Portugal joined that movement relatively late, and with remarkable material.

Much of that material came from the religious houses. In the 1830s, Portugal dissolved its monasteries and convents, and their paintings, silver and vestments passed to the state. Here is a detail many visitors miss. The newer wing of the museum stands on the site of a former Carmelite convent dedicated to Saint Albert. Its small chapel survives inside the museum complex, with gilded woodwork and blue-and-white tiles. You walk from a gallery straight into a convent's prayer space.

Now to the work that Portuguese schoolchildren know by sight. The Panels of Saint Vincent are a set of six large paintings from the fifteenth century, usually attributed to the court painter Nuno Gonçalves. They show dozens of figures gathered around a saint, standing shoulder to shoulder. There are nobles, knights, clergy, fishermen and a man holding a rope. Their faces are individual and serious. Nobody looks idealised. Many look tired.

And here is the surprise. No one knows for certain what these panels show. Historians still argue about who the figures are, which saint stands at the centre, and how the six panels were first arranged. Some see members of the royal family. Others see the whole society of the age of the early sea voyages. The debate has lasted more than a century. So stand back, look at the faces, and form your own view. You are allowed to.

From Portuguese gravity, step into Flemish strangeness. The museum holds a triptych by Hieronymus Bosch, the Netherlandish painter of nightmares, showing the Temptation of Saint Anthony. A triptych is a painting in three hinged panels. In this one, the saint kneels quietly while the world around him fills with impossible creatures. Fish fly. Buildings have faces. Small demons carry out ordinary chores. Take your time with the corners, where Bosch hid many of his oddest inventions.

Not far away hangs a work by Albrecht Dürer, the great German painter and printmaker, showing Saint Jerome. The old scholar rests his head on one hand and points to a skull with the other. It is a meditation on mortality, painted with a draughtsman's precision. Look at the beard, the wrinkles, the hand. Dürer painted it during a stay in the Low Countries, and it later reached Lisbon. Few visitors expect to meet him here.

The collection is not only European. Lisbon was a port at the edge of a global network, and the museum shows that clearly. Look for the Namban screens. These are Japanese folding screens from around the late sixteenth and early seventeenth centuries. Namban means southern barbarians, the Japanese term for the Portuguese traders who arrived by sea. The screens show tall ships, gold clouds, and foreigners in baggy trousers. Here the Portuguese are the strangers, seen through Japanese eyes.

That exchange of looks runs through many rooms. You will find Indian and Chinese objects made for Portuguese buyers, ivory from West Africa, and furniture that mixes Asian woods with European forms. Some of these pieces were luxury goods. Others carry harder histories of trade and empire. The objects themselves are beautiful, but it is worth asking where the materials came from, who carved them, and who carried them across the oceans.

Now for gold. The Belém Monstrance is a small, intricate vessel made in the early sixteenth century to display the consecrated host in church ceremonies. It is attributed to Gil Vicente, a goldsmith of the royal court. Tradition holds that its gold came from tribute brought back from East Africa on one of Vasco da Gama's voyages. Look closely. Tiny figures of apostles kneel in the structure, and an armillary sphere, a navigator's symbol, appears in the design.

The museum also keeps a deep collection of silver, ceramics, tiles and textiles. Some rooms feel like an aristocratic house that has kept its tableware. Others are quiet and almost empty of visitors, even on busy days. This is one pleasure of the place. Crowds gather at a few famous works, while whole galleries of chairs, chalices and embroidered vestments wait for anyone curious enough to wander. Let yourself get slightly lost.

When your eyes tire, go to the garden. It sits on a terrace behind the palace, with trees, statues and a small café. From here, the view drops over the rooftops toward the river and the docks. Cranes, ferries and container ships move slowly on the water. It is a working view, not a postcard. Many locals come only for a coffee on this terrace, and it is easy to see why.

Before you leave, stand on the terrace for a moment. Behind you are the faces of the Saint Vincent panels, Bosch's demons and Japanese gold. In front of you is the same river that carried the ships which made that collection possible. Listen to the gulls and the traffic on the riverside road. Then walk back out onto the Street of the Green Windows, and look up at the shutters as you go.