The Normans had added the principle of building Motte & Bailey Castles as the essential part of their plans for conquest.
Ideally, there was a castle within one day's march of each other offering defended shelter. Once built they only needed a small detachment to protect them.
Earth, timber and conscripted labour was always readily available Indeed a Motte & Baily was shipped across to Hastings in 1066. Was this the first 'flat pack'?