In particular I was wondering about earlier Medieval castles built during The Anarchy - many of which were done without Royal Permission to such an extent that it became a serious concern.
Now understandably these were much simpler Motte and Bailey castles as opposed to the towering stoneworks we often see in fantasy and historical fiction but I was rather surprised at so many being built in a relatively short period of time.
Specifically a source of conflict between King Stephen and John Marshal was the building of a castle or fort at Newbury that was able to withstand King Stephen's army for some time. Could Stephen not have done something to interrupt the creation of the castle rather than be forced into a siege or by some combination of the slow speed of information and the pace at which these castles could be built mean that by the time he knew about it there was little he could do to stop it?
A basic earth and timber castle could be - and indeed often was - built very quickly. William the Conqueror's army threw up a partially prefabricated fort at Pevensey on the very first day after landing. When working with timber and earth, labor, rather than expert craftsmanship or high quality materials, is the primary requirement. There's just not that much to a motte and bailey: it's a ditch, a palisade, and a mound of piled dirt. From all that I've seen, lords in mid-12th century England retained a considerable ability to compel service by their tenants. While I have no specific information on the construction timeline of John Marshal's castle at Newbury - which, incidentally, was not actually at Newbury, but several miles away at Hamstead Marshall - it was a motte-and-bailey castle, and so I doubt it would have taken very long to establish. Weeks, rather than months, would be my guess.