The first mention of the town is in 673 A.D., the first Synod of all the Bishops in England was held in Hertford.
In 1563, the Parliament of England met at the castle because of an outbreak of plague in London.
Haileybury and Imperial Service College, is a British independent school founded in 1862 on 500 acres of parkland occupied until 1858 by the East India College. The East India College was founded in February 1806 as the training establishment for the British East India Company (BEIC). In 1942, Haileybury absorbed the Imperial Service College, which had itself subsumed the United Services College.
In 2004 it was revealed that there is a network of tunnels underneath Hertford:
A local newspaper in the UK has revealed an elaborate network of secret tunnels and passages beneath Hertford, the ancient county town of Hertfordshire. Local historians were aware of only one part of the network which was mapped in a document on public record dated 1898. The newspaper details numerous tunnels, many of which were blocked during the last century, and shows photographs of two other entrances. Emanating from the dungeons of Hertford Castle and from Fore Street, the passages run like a warren under the ancient county town and stretch from Bluecoats to County Hall, according to the group. The warren was used by the Knights Templar, a society of warrior monks believed to be the keepers of the Grail and the Ark of the Covenant. At the heart of the maze of tunnels is Hertford Castle. It was there, in 1309, that four of six Templars from Bengeo were imprisoned following their arrest by King Edward II, who believed they were holding a lost treasure.