The game is based around Norse myth and the programmers are from Norway. Ancient Norwegian defense was a trelborg. A settlement that was ringed by an earthen berm that was in turn circled by a dry moat with gates at each cardinal point and a draw bridge over the moat at the gate.

The game gives you all the pieces that you need to set up a trelborg (with a bit of creativity) and it will keep all ground raids out. Keeping live stock inside a barn will protect them from air attacks long enough for you to deal with the air attacks.

You can also mod the game to eliminate raids if building hardened defenses it too boring for you.