elloi

"門當”與“戶對”是古民居建築中大門建築的組成部分，這種用於鎮宅的建築裝飾現今存留不多了。有“戶對”的宅院，必須有“門當”，這是建築學上的和諧美學原理。

From construction engineering point of view, the 門當 was used to strengthen the upper door frame, such that the door frame could support the weights of the bricks & roof structure on top of the door entrance.
Its decorative as well as functional.
Very clever design !