The city as we know it today was built in the early 1600s. The original town of Derry (Doire) was granted to the London craft Guilds for settlement in an effort to import loyal (and Protestant) peoples from the British mainland and thus secure the British backdoor from invasion from the continent. The town was renamed Londonderry. The settlement was attacked by local forces and the Guilds decided to build a fortified town to resist further attack. The fortified town withstood three further attacks the last of which was the one remembered to this day as 'the Siege of Derry'. Further information about this event can be found on page 6 of the Street Art section of this site.
These are two information posters found in the Guildhall Square.
The walls survive to this day, I have been told that this is the only place in Europe where the complete city walls remain intact. There have been changes, the old watchtowers have mostly gone and additional gates have been constructed over the years but substantially they are just as they were and some of the original cannon are still in place.
These cannon point out over the Guildhall Square. At the time of the Sieges the river came almost to the walls with just a small area of quayside on the outside. You can see this if you look carefully at the right of the map above.