The Jewish Temple in Jerusalem was surrounded by a fence (balustrade) that was about 5 ft. [1.5 m.] high. On this fence were mounted inscriptions in Latin and Greek forbidding Gentiles from entering the temple area proper (image below).
One complete inscription was found in Jerusalem in 1871 and is now on display on the third floor of the “Archaeological Museum” in Istanbul.
The Temple Inscription warning Gentiles not to proceed beyond this barrier—on threat of death. Istanbul Archaeological Museum.
The Greek text has been translated: “Foreigners must not enter inside the balustrade or into the forecourt around the sanctuary. Whoever is caught will have himself to blame for his ensuing death.”
The first century Jewish historian Josephus mentions the barrier and inscription in two places:
(193) When you go through these [first] cloisters, unto the second [court of the] temple, there was a partition made of stone all round, whose height was three cubits: its construction was very elegant; (194) upon it stood pillars, at equal distances from one another, declaring the law of purity, some in Greek, and some in Roman letters, that “no foreigner should go within that sanctuary;” for that second [court of the] temple was called “the Sanctuary;” (Josephus Jewish War.5.5.1 [193–194]
(417) Thus was the first enclosure. In the midst of which, and not far from it, was the second, to be gone up to by a few steps; this was encompassed by a stone wall for a partition, with an inscription, which forbade any foreigner to go in, under pain of death. (Josephus Jewish Antiquities 15.11.5 
Compare the accusation against Paul found in Acts 21:28-29:
Acts 21:28 shouting, “Men of Israel, help us! This is the man who teaches all men everywhere against our people and our law and this place. And besides, he has brought Greeks into the temple area and defiled this holy place.” 29 (They had previously seen Trophimus the Ephesian in the city with Paul and assumed that Paul had brought him into the temple area.)
There is also the possibility that this barrier is referred to by Paul when he writes:
Eph. 2:14 For he himself is our peace, who has made the two one and has destroyed the barrier, the dividing wall of hostility,
This six-line fragment of the Temple Warning was found by J. H. Iliffe east of the Old City of Jerusalem wall—near the Lion’s Gate.
Translation of the inscription from Elwell, Walter A., and Yarbrough, Robert W., eds. Readings from the First–Century World: Primary Sources for New Testament Study. Encountering Biblical Studies, general editor and New Testament editor Walter A. Elwell. Grand Rapids, MI: Baker, 1998, p. 83. Click Here to view for purchase from amazon.com.