How can I defend my sanctuary?

The only way to stop attacks is with a shield. You can get these from rewards, alliance shop etc. You can make yourself stronger at defending with research, commander talents, build traps, train troops, Nova military skills, upgrade heroes.

Joining a strong alliance and moving to their territory will deter many attackers. Try to use any resources up so that you do not have anything (over your protection level) for others to steal. This will make you less of a target.