I am starting in a game of Pendragon RPG, which takes place a generation after the Romans evacuate and leave the isles to their own defenses. Standard defensive manors are Motte and Bailey. But I have always been taught that M&Bs showed up after William the Conqueror showed up in 1066.
So, did M&Bs exist previous to Williams arrival, and he just popularized them? Or did some other sort of defensive fortification exist prior to the Saxons taking over?
There is some evidence for motte and bailey castles pre-1066, but these date from the 1040s and were built by Normans brought to England by Edward the Confessor to enforce the Welsh marches.
The most significant Anglo-Saxon fortification was the burh but these took many forms, and also didn't come into use until the mid-late 9th century (although some were built on an earlier system), so several hundred years after an immediately post-Roman setting. Many of the burhs in Wessex and Mercia, as well as attendant fortifications, made use of revamped Roman walls, while others were defended by a series of ditches and pallisades.
In the immediate post-Roman setting, Roman defensive walls were likely to still be mostly intact, but in some areas, a move would have been made towards the occasional reoccupation of pre-Roman hillfort sites as well.