More formally known as the Cathedral Church of St Deiniol, this building occupies one of the oldest ecclesiastical sites in Britain, dating from AD 525 when the saint founded his community here. The earliest part of today's stone church dates to the 12th century, although that building was largely destroyed in 1211 during a raid by England's King John. Much of the architecture seen today is the work of the eminent architect Sir George Gilbert Scott between 1870 and 1880.

Further ravages took place in the 13th century, during Edward I's invasion, and in 1402, during the Glyndŵr uprising. Two centuries later Cromwell's men used the cathedral as stables.

The cathedral has a small shop and an exhibition about its history. An artistic highlight is the late-15th-century, almost life-sized, oak carving of Christ, seated and shackled in the moments before his crucifixion. A stroll in the adjoining Bible gardens is pleasant, despite them being plundered by thieves (they previously contained examples of all plants mentioned in the Bible).