Walking the full loop of Derry's walls takes about 30 minutes at a relaxed pace, but you'll probably stop constantly. The walls are up to 9 metres thick in places and were built between 1613 and 1619, making Derry the last walled city to be built in Europe. They have never been breached, which is part of why their completeness feels significant. From the top, you look down into the Bogside where the murals are visible below, across the Foyle to Donegal hills, and along the river in both directions.
Free, open all the time, and better than any museum in the city for giving you a sense of how Derry is laid out and what it means. The walk is fully accessible from multiple entry points including from Guildhall Square. The stretch above the Bogside near the Free Derry Corner is the most historically charged section. Go in the morning before it fills up.