The Iron Age hill fort was constructed around 400BC and was used for defence for around 300 years. Cissbury is a univallate fort, that is a hilltop enclosure with a single rampart accompanied by a ditch and a low counterscarp bank. The hill fort encloses around 26 hectares and originally had only two entrances, one at the eastern corner and the other at the southern end.
After 100BC the interior of the fort was used for agriculture with rectangular fields being marked out with earthwork banks and terraces.
There is archaeological evidence of a settlement at Cissbury during the later Roman period. This comprised a group of 11 buildings and two rectangular enclosures situated near the eastern entrance to the fort. The ramparts were heightened at this time possibly in fear of Danish attacks.
The discovery at Cissbury of two successive issues of coinage struck between AD1009 and AD1023 suggests that there was once a mint here.
There is little evidence of activity here during medieval times but many of the local agricultural trackways across the downland are likely to have originated during this period.