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Fausset's Bible Dictionary [1]

("house of caverns".) Two towns, the upper and the nether, separated half an hour's journey; now Beitur et tahta and Beitur el foka. On the road from Gibeon (now el Jib) to Azekah and the Philistine plain ( Joshua 10:10-11;  Joshua 16:3;  Joshua 16:5;  Joshua 18:13-14), on the boundary between Benjamin and Ephraim, but counted to the latter and given to the Kohathites ( Joshua 21:22). Sherah, a granddaughter or descendant of Ephraim, built (i.e. enlarged and fortified) both the upper and nether Bethhoron, and was of the family whence sprang Joshua ( 1 Chronicles 7:24;  1 Chronicles 7:27). (See Sherah and (See UZZEN-SHERAH.) Here Joshua conquered the five kings of the Amorites.

On the mountain S. of the nether village (Ajalon) over which the sun stood still there remains still the name Yalo. From Gibeon to upper Bethhoron is a distance of four miles, partly descent, but mainly ascent; hence it is called the "going up" to Bethhoron ( Joshua 10:10-11), but in the second stage of Joshua's pursuit it is the "going down to Bethhoron," the descent beginning from the upper village toward the lower one. This has been for ages the road of communication for heavy baggage between Jerusalem and the Philistine sea coast; it goes W. to Gimzo (Jimzu) and Lydda (Ludd), where it parts into three, the N. to Capharsaba (Antipatris), the S. to Gaza, and the W. to Joppa (Jaffa). Hence, as the route is key to a large part of the country, Solomon fortified both villages ( 2 Chronicles 8:5). Still great foundation stones are visible.

Morrish Bible Dictionary [2]

Two towns called the 'upper' and the 'nether,' though also spoken of as one, on the boundary between Benjamin and Ephraim. They were allotted to Ephraim, and given to the Kohathites. The district is memorable as where Joshua conquered the Amorites, and near which God smote them with hailstones.  Joshua 10:10,11;  Joshua 16:3,5;  Joshua 18:13,14;  Joshua 21:22;  1 Samuel 13:18 . In  1 Chronicles 7:24 these towns are said to have been built by Sherah, apparently the grand-daughter of Ephraim. Solomon also built or rebuilt them.   1 Kings 9:17;  2 Chronicles 8:5 .

It was near these cities that Judas Maccabaeus won his victory over Seron; and here that the Roman Cestius Gallus was signally defeated. The places are still called upper, el Foka, and lower, et Tahta, with the general name of Beit Ur , 31 53' and 54' N, 35 6' and 5' E.