I build barns (completely covered) for my live stock. If the event creatures can't see the livestock they won't attack them (unless they make a hole in the barn but I'm usually there defending by the time they can take out a bit of wall.)

What the actual Vikings did was build Trelleborgs. The Trelleborg was an earthen fortress. They were circular and had an earthen berm built up all the way around it and out side of that a dry moat. There was a gate through the berm at each cardinal direction point with a draw bridge to cross the moat. Roads made a cross in the exact center and each section inside the Trelleborg had a group of 4 wooden lodges in it with an open square in the center of each group of 4 lodges for outside work.

The berm with the moat works to keep ground creatures out but since the game lacks drawbridges you have to defend your gates (the tolls can destroy them by throwing rocks) and you have to build a bridge and then dismantle it when you are done any time you want to take a cart in or out.