If a yard is over 50% destroyed, it will be under 12 hours of "damage protection", during which the yard cannot be attacked until either the time runs out or the protected player attacks another player, which removes the remaining protection time.
It will be under 12 hours of "damage protection". A yard is over 50%. This 50% is destroyed. The yard cannot be attacked until either the time runs out or the protected player attacks another player during 12 hours of "damage protection". Another player removes the remaining protection time.