The Cathedral of St Petri is a twin-towered early Gothic church that has stood in various forms since the 8th century, though the present structure dates mainly from the 11th to 13th centuries. The interior is a mix of Romanesque and Gothic periods, with a fine organ and several historically important monuments. You can climb one of the towers (April to October) for views over the Altstadt and the Weser.
The real draw for many visitors is the Bleikeller in the basement crypt — a macabre and unexpectedly compelling attraction where eight mummified bodies have been displayed since the 18th century. The preservation is natural, a result of the lead-lined chamber's unusual microclimate. Combined with the main church and the tower climb, the Dom easily fills an hour.