Hello, the hole in the rock wall of Yungang Cave is used to fix wooden beams. The scientific name of the grottoes is "Cave Temple", which means that there are temple buildings in front of the grottoes. Now there are no temples, only grottoes. For example, in Tan Yao No.5 Cave (Cave 16-20) excavated in 460, the stone wall was covered with regular square holes. There is a magnificent ancient building in front of the cave, and the beam of the building is connected with the stone wall. The building no longer exists, but the square hole still exists. The same example can be seen in Fengxian Temple in front of the giant Buddha in Lushena, Longmen. There are abundant records about Fengxian Temple in historical materials. The Buddha statue of Lushena is the main statue enshrined in the last courtyard and hall of the temple. At present, there are still some caves on both sides of the Rushena Buddha (see photo).
Another situation is that there are cloisters or eaves outside the grottoes, so the caves are fixed, which can be seen on the eaves of the Song Dynasty remains in Dunhuang Mogao Grottoes. The picture cannot be uploaded. I posted it on Sina blog: Xiyan Studio.