All buildings except for the Tier 1 Towers possess the passive ability backdoor protection. Backdoor protection activates when there are no nearby enemy creeps, and causes the building to take 25% damage from illusions and 75% damage from other units. If a backdoor-protected building is damaged by an enemy, it will regenerate the lost health at a rate of 90 HP per second. It is still possible to destroy protected buildings with sufficiently high damage. Backdoor protection will not regenerate damage dealt by friendly units.
On the Tier 2 Towers, backdoor protection is independent of each tower. On the Tier 3 towers, however, the faction base applies backdoor protection as one single unit. E.g. if there are enemy creeps near one Tier 3 Tower, all the other Tier 3 towers will drop their backdoor protection as well. Barracks are invulnerable until their guarding tower is down, but even if the guarding tier 3 towers are down, the pair of barracks have backdoor protection until the enemy creeps come close.