From "The Doorways of Solomon’s Temple" by Megan Sauter:
King Solomon’s Temple was resplendent. Described in 1 Kings 6–7, the temple was divided into three parts: the forecourt (ulam), the outer sanctum (heikhal) and the inner shrine (devir), also known as the Holy of Holies. Built of stone and roofed with wooden beams, Solomon’s Temple was intricately ornamented. Its interior walls and floors were lined with wooden boards and covered in gold. It took seven years to complete the temple and its furnishings.A nearby temple of the similar period located just 20 miles away from the Temple Mount in Jerusalem — Khirbet Qeiyafa — may hold the answer to this curious description and the enigma of "five doorposts." That temple is known to have been occupied in the 11th-10th century B.C., and was a fortified site in Judah from the time of King David.
Despite the Biblical description and archaeological parallels, there are still some mysteries about Solomon’s Temple. For example, 1 Kings 6:31 describes the doors between the outer sanctum and the inner shrine of Solomon’s Temple as having five mezuzot (the plural form of mezuzah). What is a mezuzah? In the Bible, mezuzah is normally translated as “doorpost.” However, in the context of Solomon’s Temple, doors with five doorposts do not make sense...
[Researchers] Madeleine Mumcuoglu and Yosef Garfinkel present a building model from Khirbet Qeiyafa that depicts a doorframe with three—or maybe four—interlocking doorframes surrounding an opening. They suggest that these interlocking frames are what is meant by mezuzot in the description of Solomon’s Temple. According to their interpretation, then, the doors to the inner shrine (devir) of Solomon’s Temple did not have five doorposts—but rather five recessed doorframes.
Temples with recessed doorways have been uncovered throughout the ancient Near East, and Madeleine Mumcuoglu and Yosef Garfinkel further note that recessed doorframes set apart inner rooms and spaces as being sacred. In the case of Solomon’s Temple, the more recessed the doorframes, the holier the space:
"There is a gradual increase in the number of recessed doorframes from the entrance to the forecourt (three) to the outer sanctum (four) and finally to the entrance from the outer sanctum to the devir (five). It is as if the devir had the highest number of mezuzot because it was the most sacred part of the Temple."