Rule 310.4 · Battles
Defense is a characteristic that battles have.
Subrules
The defense of a battle card not on the battlefield is equal to the number printed in its lower right corner.
A battle has the intrinsic ability “This permanent enters with a number of defense counters on it equal to its printed defense number.” This ability creates a replacement effect (see rule 614.1c).
The defense of a battle on the battlefield is equal to the number of defense counters on it.