Robin Hood - Sherwood Builders

On the world map N / NW of the starting village there’s a gray house / stone building icon, and others on the map. Go here and look for a small cave with white marble entrance. Inside there are four pressure plates on the ground step on them on order: top left, bottom right, top right, bottom left. A wall to your left will drop revealing a secret room with minor treasure and achievement will unlock.

This should unlock at any of the puzzle building icons but I explored this one first.