Ireland Has a Unique 'Derry Wall' Feature - Apparently.
The city walls of Derry, Northern Ireland, have a distinctive feature: 16 of its 17 gates are still intact, but only one of them, the Bishop's Gate, has a unique 'sally port' feature. This small door, built in 1618, allowed the city's defenders to exit and re-enter the city without opening the main gate, providing a secret escape route. The Bishop's Gate sally port is a rare example of this architectural feature, and its existence has been verified by local historians and architectural experts.