Greencastle is dominated by Northburg Castle. Construction was started in 1305 by Richard De Burgo, 2nd Earl of Ulster. This was the main Anglo Norman stronghold in the north west and was reputedly based upon the design of Caernarfon in Wales. A strong ‘curtain wall’ enclosed a lower and upper ward with a timber great hall. The power of the Normans in the area was fatally weakened by the campaign of Edward the Bruce of Scotland in 1315-18 however and De Burgo had lost control by 1322. To one side of the castle a tower house was constructed by the O'Doherty’s in the fifteenth century.
