Originally this was a term for a game-park (from Old French garenne) and was used for areas where a right of ‘free warren’ (the liberty to hunt small game) had been granted. By extension this came to mean an area set aside for the keeping and breeding of rabbits for their meat and fur. Rabbits were an early Norman introduction (though there is some debateable evidence for an earlier and possibly short-lived Roman introduction) from the Mediterranean and rabbit warrens are documented from the 12th century onwards. Medieval documents distinguish between cuniculi, adult rabbits (hence the term coneys) and rabetti, the young rabbits.
The warrens were normally defined by earthen banks to keep the rabbits in and to keep predators, human and otherwise, out. In some cases mounds were provided to encourage the rabbits to form burrows and lodges were built for the warreners who guarded and managed the rabbits. Many warrens were still under active management in the 19th century, but became disused in the 20th century.
In Suffolk there were large warrens in the sandy soil areas of the Breckland and the Sandlings (qv). One of the largest, Lakenheath Warren in Breckland, covered over 2,300 acres in 1835 and was to have stock of 7,200 rabbits. In Breckland, many of the early large warrens where established by ecclesiastical landlords: the prior and convent of Ely received the right of free-warren over their land at Lakenheath in 1251 and a specific grant of a cunicularum was awarded in 1300; the bishops of Ely had a warren at Brandon by 1252; and Bury abbey had a warren at Mildenhall by 1328.