Church architecture is the backbone of European city centres, and most visitors walk past the best interiors on their way to something else. These five cities have churches that are worth the trip in their own right, and each offers something architecturally distinct from the others.

The Cities

Rome

Rome has more significant churches per square kilometre than any other city in Europe, and the range spans 1,500 years of Christian architecture. St Peter's Basilica in the Vatican is the obvious centrepiece, but the more rewarding visits are often the lesser-known basilicas: Santa Maria Maggiore (4th-century mosaics), San Clemente (three layers of history stacked vertically underground), and Santa Maria in Trastevere (glittering 12th-century apse mosaics). Entry to most Roman churches is free. The key detail to look for inside: the Cosmatesque marble floor patterns, the choir screens, and the apse mosaics that survived the Byzantine and early medieval periods.

Best for: travellers who want the broadest range of Christian architecture across the longest time span, most of it free to enter.

Barcelona

Sagrada Família is the strongest argument for visiting any single building in Europe for architectural reasons alone. Gaudí's unfinished basilica, under construction since 1882 and still incomplete, combines Gothic structural logic with Art Nouveau organic form in a way that is genuinely unlike anything else in existence. The interior in particular, completed to modern additions of the nave in 2010, is extraordinary: the branching columns, the coloured light through the stained glass, the upward geometry. Book tickets far in advance. Beyond Sagrada Família, the Gothic Quarter contains the Barcelona Cathedral and several smaller medieval churches worth the detour.

Best for: travellers for whom a single building justifies the trip, and who want to see architecture at the edge of what the discipline has produced.

Cologne

The Cologne Cathedral (Kölner Dom) is visible from almost everywhere in the city and was the tallest building in the world for four years after its completion in 1880. The Gothic structure took over 600 years to complete, and the scale inside is genuinely disorienting: 144 metres long, with the nave climbing 43 metres overhead. The Shrine of the Three Kings behind the high altar is the reason the cathedral exists: it has been drawing pilgrims since the 12th century. Entry to the main cathedral is free; the treasury and tower climb require tickets.

Best for: travellers who want pure Gothic scale and a cathedral that has functioned as a pilgrimage destination for nine centuries.

Vienna

Vienna's Stephansdom (St Stephen's Cathedral) anchors the city's historic centre and rewards both exterior and interior attention. The Gothic south tower, the geometric tiled roof visible from above, and the interior nave with its Baroque accretions provide a layered architectural experience. Peterskirche, a few minutes' walk away, is a compact Baroque church whose interior is disproportionately dramatic relative to its small exterior. The Karlskirche near the Naschmarkt is early 18th-century Baroque at its most confident. Vienna's churches collectively cover Gothic, Baroque, and neo-Gothic in a dense central area.

Best for: travellers who want Gothic and Baroque church architecture in close proximity, with the Stephansdom as the anchor and several strong secondary churches nearby.

Reykjavik

Hallgrímskirkja is an outlier on this list in every sense. The Lutheran church was completed in 1986 and is based on a design that references the basalt lava columns found across Iceland: the facade rises in stepped concrete columns that echo the landscape rather than any European church tradition. It is the most architecturally original church built in Europe in the 20th century, and the contrast it provides to the medieval and Baroque churches of the continent makes it worth the comparison. The tower has a lift to an observation deck with views across the city.

Best for: travellers interested in church architecture as a wider discipline rather than specifically medieval or Gothic periods.