Well, I would more agree about feudalism origin through ancient Roman social relationship where a client (mostly a plebeian) was sponsored by a patron benefactor. The patron assisted his client with his protection and regular gifts; the client dedicated his vote whenever the patron or his associate was up for election. By degrees, the custom extended itself beyond Rome; and not only families, but cities, and entire provinces.However, the difference between the relation of vassal and his lord, and that of clients and his  patron, is fairly considerable and not only because of a Germanic background but in addition to the respect a client showed his patron, and the vote he gave him, a vassal was also obliged to assist his lord in all affairs; and even pay his ransom, if he should be taken prisoner of war, in the case that the lord was not able to provide it himself. As Phidippides  said, the emergence of feudalism during the High Middle Ages was greatly    favoured by the weakness of a strong centralized power.And as Scout said, Salic law was probably more about inheritance than political regime and the roots of feudalism are to be searched in ancient Rome. 🙂

I agree that when William moved to Hastings, he built a prefabricated wooden castle for a base of operations. And yes, it can be seen in the Bayeux tapestry when William ordered defence dugs around the Hasting “ceastra”.

Well, the Normans were from Viking lineage, they kept an efficient navy even if England didn't have one at the time. The reason why they could settled in Normandy was that they were there to protect West Francia against other Norse ppl. That fleet was not for transportation only, I think.  About the prefab fort, where do you see that ? There is enough in England to build a motte-and-bailey.

Well another explanation can be that the Carolingians didn't have any Navy to secure their coasts especially against the Vikings… Am I wrong ?