How do I set up a defense team?
Defense teams can be set in the Defense tab of the PVP menu, shown below.
Your defensive strategy is entirely in the hands of the AI, so it is to your advantage to make the team easy to use automatically and not provide a lot of opportunities for choice that the AI can misuse. If you want to win, that is.
A basic example: rather than using a troop that transforms one color into another (where the AI can select a poor choice of color to transform), instead use a troop that simply creates a bunch of gems on the board.
Avoid using complicated teams that require a specific troop to be targeted by another in order for it to function. The AI won't always do what you want it to.
What do I get for using a defense team?
If your defense team wins, you earn a small amount of gold, souls and glory (as well as the occasional Gem or Gold Key) when you next visit the PVP menu. If your defense team loses, the player who beat you will be flagged as a Revenge target and be worth additional glory if you beat them in return.