Saint Bavo's Cathedral is the main reason to come to Ghent if you have any interest in art history. The cathedral houses the Ghent Altarpiece — formally "The Adoration of the Mystic Lamb" — painted by Hubert and Jan van Eyck and completed in 1432. It is one of the most important paintings in Western art, a polyptych of 12 panels that broke with the flat Byzantine tradition to achieve a new kind of pictorial depth and realism.
The cathedral itself is Gothic, built over four centuries between the 10th and 16th. The interior is large and dark and the altarpiece is displayed in its own climate-controlled room, visible for an additional entry fee. The restoration now ongoing is revealing details hidden under centuries of varnish, and certain panels remain in the conservation studio — check the website for current status. Even in partial form, the altarpiece is extraordinary.