Short answers, pulled from the story.
The Trinovantes established their capital at Colchester during the Iron Age. This settlement became Britain's oldest recorded town and featured its own mint producing coinage that marked them as one of the more advanced tribes on the island.
Colchester was taken by the Roman army after they landed on the south coast in Richborough area of Kent. The city became a Roman Colonia named Colonia Claudia Victricensis and housed the largest temple to the God-Emperor Claudius in Roman Britain.
Sledd is recognized as the first known king of the East Saxons in Essex in 587. Less reliable sources attribute the founding of the kingdom to Aescwine in 527, but Sledd remains the primary historical figure for this period.
The Peasants' Revolt broke out in Brentwood on the 1st of June 1381. Several thousand Essex rebels gathered at Bocking on the 4th of June before dividing their forces to head toward Suffolk and London.
The present boundaries of Essex were set in 1998 when Thurrock and Southend-on-Sea were separated from the non-metropolitan county to become unitary authorities. This followed earlier administrative changes including the formation of Essex County Council in 1889 and boundary adjustments with Greater London in 1965.