St Columb's Cathedral stands within the city walls in the south-west corner of the old city, built from the same grey-green schist as the walls themselves. It is the cathedral of the Church of Ireland's Diocese of Derry and Raphoe, and at nearly four centuries old, it is the oldest surviving building in Derry.
The interior is notable for several reasons. The chapter house holds artefacts from the Siege of 1689, including the padlock and key from the boom that the Jacobite army stretched across the Foyle to prevent relief ships entering the city. The original lock was reportedly given to the enemy as a provocation, and its return was a condition of the siege's end. The cathedral also has a fine collection of 19th-century stained glass and monuments to notable parishioners. Entry is by a small suggested donation.