The Citadel stands at the southern end of English Street, two round towers that look medieval but were actually rebuilt in the 1810s to designs by Thomas Telford, who replaced the original Henry VIII artillery fortification with something more imposing. The towers were built as law courts — they served that function until 2011 — and the interiors are not routinely open to the public, but the exterior is worth a close look.
What the Citadel really does is frame the approach to the city from the railway station. Walk north from Carlisle station along English Street and the twin towers come into view ahead, framing the city centre beyond. It is a better arrival experience than most English cities offer, and the sight of them in the evening light gives you some sense of why Carlisle took its role as a border stronghold seriously.