What happens when an attacker's unit survives combat but cannot deal damage due to Rune Prison, and the defender's unit also survives?

Ruling: When the attacker is unable to gain control of the battlefield (because their unit cannot deal damage due to Rune Prison and the defender's unit survives), the defender retains control and the attacker's unit is recalled to base.

Sequence:

• Combat occurs with Rune Prison preventing the attacker's unit from dealing damage
• Defender's unit survives (not enough power to kill attacker's unit)
• Attacker fails to gain control of the battlefield
• Defender retains control
• Attacker's unit returns to base