Groß-St. Martin is one of Cologne's great Romanesque churches, its heavy crossing tower rising above the lanes of the Old Town near the Rhine. The church began as a Benedictine monastery in the 10th century, while the present form is mostly 12th and 13th century. It is worth seeing for the scale of the tower, the trefoil choir, and the way the stone interior feels calm after the crowded riverfront outside.