In your castles and outposts (this includes the castles in other kingdoms, like Everwinter Glacier, The Burning Sands, Fire Peaks and The Storm Islands) you can set your defenses.
The best defense, is to tailor to your defense to counter what the incoming attacker has sent. That’s not the topic of this post. In this tip, I’ll explain how to set your defense for when you are offline. The “Best Guess” method to handle most attacks.
First: Do NOT defend all three walls. Pick one (left or right). If you remember from the Courtyard Bonuses tip, you get different bonuses or penalties depending on how many walls you defeat. It is easier to defend a single wall (and hold it) than to defend all three. So try to hold one wall, which will prevent the attacker from get the Courtyard Bonus.
Second: Setup your Unit Distribution to use the 80/20 rule. 80% Melee soldiers, 20% Ranged. The reason for this, is that there are a lot more tools to decrease Ranged Defender effectiveness than to decrease Melee Defender effectiveness. Note: You need to set this for EACH WALL! 
Third: Use the best tools you can afford. Some tools cost resources, others cost rubies. The more rubies, the better the tool. Use either the slit/slit/mach rule or the mach/mach/slit rule. These are the “Arrow Slit” and “Murder Hole” tools. If you can’t afford Ruby Tools, then use what you can afford (Flaming Arrows and Hurling Rocks). If you have Moats, used the best Moat tool available (usually Fire Moat).
(NOTE: The Murder Hole used to be called machicolation before being renamed).
That’s it. This is not a guaranteed “always win” setup. However, it will help keep you safe while offline. As always, keep the best melee/ranged gear on your castellan that you can.
TIP: If you are an active player, you can run your food in the red. By actively attacking player and robber baron, you will loot more food than you consume. This lets you keep more defensive troops in your castle than your food normally supports.