The three settlements that England possesses at the start are actually London, Nottingham and Caen.

London is a town, while Caen and Nottingham are Castles.

The rebel held settlements in the British isles are:

York (Town)
Caernarvon (Castle)
Dublin (Town)

York can be captured quickly as it has no wall defences and only a few units of light infantry, and Dublin is similar except it has slightly more and better troops. Caernarvon is a Wooden castle that requires the walls to be breached by catapult, or assailed using ladders. The catapult is the best option as the Welsh have longbows who can decimate your assault siege equipment before it reaches the walls, while the catapult can fire from a safe distance.