Runes are the main resource used in Spell Casting.
Obtaining Runes
Towers generates additional runes per Utopian Day.
Runes can also be taken from an enemy province, via:
Decay and Destruction
After each tick, 1.2% of the total runes in storage will decay.
An enemy province's rune supply can be attacked via the Lightning Strike Spell. This attack will not give the destroyed runes to you.