Level up your wall and train defensive units (click on the wall to do this).

Build up your might and your army (see CONQUERING WILDERNESS ) so people won’t be willing to attack your city. 

Set the troops to defend (in your castle).

Find a tough alliance (see ALLIANCES)
who will stand by you if you are attacked.
Of course, defenses always fight so aren’t hard to wipe out when you don’t have an army to back them up, and leaving your troops on defend DOES run the risk that somebody bigger could come along and wipe your army out while you’re away….
I suggest focusing on the Hunker Down approach.