Backdoor protection

Backdoor protection is a special passive ability of structures that is meant to prevent sieging towers without help of allied lane creeps. All structures have this except for tier 1 towers. This ability will regenerate the attacked building at the rate of 90 HP/s if damaged by enemy heroes without allied lane creeps or their corpses around, and reduce the incoming damage by 25%. Lane creeps will disable this ability of enemy buildings they get into proximity. It will stay disabled as long as there is a corpse of lane creep or any enemy heroes near. Backdoor protection of structures in base is disabled on the whole base as opposed to tier 2-3 towers. With some decent attack speed and damage it's still possible to beat backdoor protection, especially with a team of 5 heroes on the same tower. This ability is meant mainly to prevent surprise raids on the enemy towers while its team is not available to defend.

Trivia

 * In DotA the Ancients don't have backdoor protection, but in DotA 2 they do.
 * In most tournaments backdooring is prohibited.
 * Another passive ability of several structures is True Sight.