Although it was a poor country riddled with petty kingdoms, Wales was the one region in southern Britain that survived the Anglo-Saxon onslaught following William the Conqueror's victory on the island. The main stay of Wales survival came from its mountainous and wooded terrain, and the very fact that the many rivaling kingdoms within its own borders established a ready-made guerrilla warfare that the royalty of England was ill-prepared to subjugate. Because of the lack of communications once the mountain ridge border was crossed, and the difficulty of sustaining logistics during guerilla ambushes, even William I decided on a defensive posture against the Welsh, wherein he established marches along the border that were charged with preventing Welsh encroachment--by royal rights and authority to build castles, and at the same time he permitted free license to the March lords to advance into Wales.
The initial English push enjoyed adequate success from 1067 to about 1075, but as English rule spread further into Wales, the Welsh princes put aside their internal conflicts to rally and drive out the English, recovering almost fully the land loss by the English advance by the break of the 1100's.
Unfortunately for the Welsh, however, the break in the everyday disunity between Welsh overlords was short-lived, and it was this weakness which the English sought to exploit. Because the Welsh fought a "typically 'dirty' style of war" (Hooper & Bennett 71) the English were already aware that even their heavy cavalry was impotent once they penetrated the guerrilla war-friendly home ground of the Welsh defenders. Gerald of Barry, a royal servant and descendant of a marcher family, recorded his advice on subjugating Wales:
Blockade 'free' Wales by land and sea...and prepare castles in the marches and interior. English money should be used to exploit dissension between welsh...