What are the Best Defensive Tactics?

Always make sure you have maxed hospitals as you level because when you find your Command Center under attack the battle will basically come down to your hospitals and resources versus the enemy's power.

It is more beneficial to build hospitals instead of a resource because the number of soldiers you can heal in them could be gathering resources to help stave off an attack. As this is a corps fight other players should help reinforce the player getting attacked by giving them their spare resources so they can heal the the troops in hospitals. Another tactic is to deploy your troops to a corps member that has a shield on them.