The Dom St. Peter is one of the three Romanesque "Imperial Cathedrals" of the Rhineland (with Speyer and Mainz), consecrated in 1181 and arguably the best preserved of the three. Four round towers, two large domes, and a strong horizontal sandstone facade — the silhouette is essentially unchanged since the 12th century.
Inside, an unusually plain Romanesque nave gives way to a wildly baroque Balthasar Neumann altar at the east end — a startling contrast that explains much of how German church interiors evolved. The choir stalls are early 17th century, and the medieval tombstones along the south aisle are worth working through one by one. Free entry; modest charge for the small treasury.