Keep in mind of a couple of things.

It is a survival game, not a building sim. And it is in early access. Developers have hinted a possible difficulty level setting in the future. This could include a creative mode. But it could easily be years away.

In the meantime, there is a debug mode where you can have full access to building items, etc. This also include a keystroke command to kill anything near by. So if you build in that mode and get a raid, you can kill them off easily without worry.

How:
Open console: Press F5 key
Enter: devcommands
Enter: debugmode
Close console: Press F5 key again

Keystrokes during debugmode on:

B to toggle no material loss
K to kill all monsters around you
Z to toggle fly mode (careful, if you are high up and toggle it off, you will fall and die :) )