In Smallworld, do Fortresses pay the +1 VC only on the turn they are placed, or do they each pay for every turn that the race is active?
They work at the end of each your turn (not only when they are placed).
Even if it might by ambiguous in the Smallworld rules,

Fortified
Once per turn, as long as your Fortified Race
is active, you may place 1 Fortress in a
Region you occupy. The Fortress is worth 1
bonus Victory coin at turn's end, unless you
are In Decline. The Fortress also augments your Region's
defense by 1 (just as if you had an additional Race token
stationed there), even if you are In Decline.
Remove the Fortress if you abandon the Region
or when an enemy conquers it. There can only
ever be a maximum of 1 Fortress per Region,
and a maximum of 6 Fortresses on the map.


Everywhere else, symbol +1 VC on a land or token (such as Fortress) says that it works each turn.
If it were only about the end of the current turn, then the sentence at turn's end, unless you are In Decline wouldn't make sense (as you cannot place anything when you are in decline, unless you are Stout, but you cannot be Stout and Fortified at the same time).