The very first invasion I experienced with my friends was Eikthyr rallying creatures of the forest. That was easy. This week when I started on another world all over, we never had that event even during a week's time in game time.

The second-ever event I experienced was drakes. Literally a cold shower.

It's up to you how you fortify your home. Terrain, phase of the game and personal preferences play a large part in that. You don't have to dig a moat if you don't want to, but then you will certainly need sturdy walls. You can lay or stack various defenses behind or on top of each other. It's up to you to find the method that works best for you. Good luck.

Edit: Greydwarves are vulnerable to fire, consider making fire arrows.