Colchester lays claim to being the oldest recorded Roman town in England, existing as a Celtic settlement before the Roman conquest. There is archaeological evidence of settlement 3,000 years ago. Its Celtic name was "Camulodunon", meaning "the Fortress of Camulos". (Camulos was the Celtic god of war.) This name was modified to the Roman spelling of "Camulodunum" (written "CAMVLODVNVM") and the town was developed as a major colonia in the early stages of the conquest of Roman Britain, possibly with a view to its becoming the province's capital. It was sacked in the Boudican revolt, and though it recovered afterwards and lasted throughout the Roman occupation, its position as capital was assumed by Londinium.
Colchester Castle is the borough's main medieval landmark. The surviving castle building is an 11th century Norman keep built in the same style as the Tower of London. Few traces of the outer buildings, walls and bailey remain. The castle is built atop an old Roman temple. The castle is surrounded by the landscaped Castle Park
In 1648, Colchester was thrown into the thick of the Second English Civil War when a large Royalist army (led by Sir Charles Lucas and Sir George Lisle) entered the largely Parliamentarian (Roundhead) town. They were hotly pursued from Kent by a detachment of the New Model Army led by Sir Thomas Fairfax, Henry Ireton, and Thomas Rainsborough. The Roundheads besieged the town for 76 days. By that time, many of the town's most ancient monuments like St. Mary's Church and the Gate of St. John's Abbey were partially destroyed and the inhabitants were reduced to eating candles and boots. When the Royalists surrendered in the late summer, Lucas and Lisle were shot in the grounds of Colchester Castle. The spot is marked by an obelisk today and there is a myth that no grass will grow in this area