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Free warren—often simply warren—refers to a type of franchise or privilege conveyed by a sovereign in mediaeval England to a subject, promising to hold them harmless for killing game of certain species within a stipulated area, usually a wood or small forest. The sovereign involved might be either the monarch or a marcher lord.
The grant of free warren could be as a gift, or in exchange for consideration, and might be later alienated by the grantee. The stipulated area might be coextensive with the frank-tenement of the grantee, or it might be discontinuous or even at a considerable remove from the grantee's holdings. The right of free warren did not extend automatically to the freeholder of the soil.
Although the rights of free warren are usually discussed in the context of forest law, the only law which applied within the warren was common law. Thus, even though the warrant ultimately derived from the sovereign, the only statutes applied to poachers in a warren were the common-law crimes of theft and trespass.
The privilege of free warren was a reciprocal relationship. The grantee of the warren was granted an exemption from the law (under which all game in the realm was property of the sovereign), but the grantee owed the sovereign the stewardship and protection of the game from all others who might wish to hunt it.
Modern English warren ← ME warrene, warreine ← ONF warrenne ← Germanic present participle of *warian "to take care; to cause to care (for)" ← causative of *waran "to care" ← *war "care". Doublet of guarantor. Related to OHG werien (i.e. *wärian) "to defend, protect", and also to English "a-ware, wary". 
The original use of free warren was as a legal term. However, as the franchise defined both a set of species and a geographic extent, the natural semantic extensions arose, namely for the individual animals as a group, or for the land they inhabited. As it became pragmatically necessary for freeholders not holding a free warren to enclose their breeding establishments, these "closed warrens" or domestic warrens began also to be designated simply as "warrens" (use recorded in 1378; OED). In 1649 the metaphoric use as "cluster of densely populated living spaces" is recorded.
(5) But the warden, as long as he hath the custody of the lands, shall keep up and maintain the houses, parks, warrens, ponds, mills, and other things belonging to them, our of their issues;
The warden of a Royal forest was often the castellan or constable of the nearest royal castle; over time the less exalted title of warrener evolved for the custodian of the lowest of the hunting franchises, the warren.
The adjective free in free warren does not refer to the lack of enclosure surrounding the precincts of the warren, but rather to the fact the "liberty" of hunting derives from a warrant of the sovereign. That is,
The term "warrant" occurs very early in constitutional documents: it is found in the Assize of Clarendon and the Assize of the Forest, both in the reign of Henry II., but in neither case in its modern meaning. The original meaning seems to have been more akin to guarantee (q.v.), warranty or security; and to some extent the term implies something in the nature of a guarantee or representation by the person issuing the warrant that the person who acts on it can do so without incurring any legal penalty.
The permission to take game was limited to certain types of animals. Generally, the killing of vermin (defined as predators and other beasts not fit for the table) was not regulated. This definition was flexible, however, depending on whether the animal was thought to provide good sport, as wolves, foxes, badgers, or bears. In practice, vermin could only be killed on the commons or waste, since none but the grantee was permitted to have instruments of the hunt within the warren.
"The beasts and fouls of Warren are these, The Hare, the Cony, the Pheasant, and the Partridge, and none other are accompted beasts or fouls of Warren."
However, Manwood is mistaken in his assignments, since the roe deer was transferred to "beast of warren" from "beast of the forest" in the fourteenth century. Roe deer are still found within woodlands named "Warren" in contemporary England. The 1911 Encyclopedia adds roe, woodcock, quail, and rail to Manwood's list. On the other hand grouse are not birds of warren. Fox, wolf, cat, badger, and squirrel are sometimes also added.