The walls (1.5km long) that surround the city were built from 1613 to 1618. Despite their robustness, demonstrated during more than one siege, they are no marvel of military engineering: they are too exposed to shots from ships on the Foyle. Starting with a walk on the eastern side where Newmarket Street crosses the walls, and going clockwise around the fortifications will give you an opportunity to admire the gates, bastions, observation towers and artillery. There are also magnificent views to enjoy.