Killing the bosses is what triggers raids, although I think you can get the real easy one of just some animals attacking before you kill the first boss.

They do not occur when you are not at your base. They can only occur where you are at.

As far as when you should build defenses, if you value what you've done, then defend it. Don't build some epic structure and then complain that it gets destroyed because you didn't defend it.

:)