I've got a question about the fortify action. The Rulebook states that
all the rest of the holdings in a province fortified are also fortified
if a castle is "built".

So if a 4th level province of a priest regent is fortified over a number
of turns t build a 4th level castle( the priest controls all the law and
all the temples as holdings) does this create level 4 law fortified
holding and a level 4 fortified temple holding?

A neighbour now invades this province, ( the rule book states that all
fortified holdings must be besieged or assaulted to neutralise them).
Does that invader have to use 12units plus a number of units equal to
guards in the castle for 1 year or can he use 4+ units for 3years! Its
just that this situation has just come up and I haven't yet made a
ruling on it yet.

Of course if the invader wanted to capture the province he would only
need to neutralize the castle, if he wanted to rule effectively he would
need to get rid of the fortified law ( 2 years)