Hi,
This is an idea I've had as well, but lacking either the Vikings' or any of the Old West forts, I've never been able to put it into practice. Bummer.
I know there are folks out there who have used the palisades from the Vikings/Old West in combination with Romans, but I can't recall having seen a set-up specifically populated with Vikings.
I also think it would be possible to incorporate the semi-ruined gate tower from the Barbarians set 4435, which I have got...
Cheers,
AndrewL