Machaerus, meaning the Black Fortress, was located on the frontier of Arabia, about 9 miles / 15 kilometers east of, and at an elevation 3,860 feet above, The Dead Sea. Originally built by The Maccabees, it was destroyed by Roman forces under Pompey, but thereafter the Romans, particularly in the time of Herod The Great (the Roman-appointed "king of Judea" who, while attempting to have the newborn Christ killed, ordered the prophesied (Matthew 2:17-18, Jeremiah 31:15) slaughter of many other infant Jews in and around Bethlehem i.e. Matthew 2:16-18) rebuilt and expanded the royal residence, military facility and prison. It was at Machaerus, according to the Jewish-Roman historian Flavius Josephus, who lived through much of the New Testament era, that the Biblically-recorded martyrdom of John The Baptist occurred in the time of Herod the Great's son, Herod Antipas (see the Fact Finder question below).
John In Machaerus
John was a fearless servant of God, that is to say, he feared only God. When Herod Antipas committed adultery (which according to Josephus, was adultery and incest), John boldly rebuked the tetrarch for his sin, just as he would have anyone else. Unfortunately, for John, this occurred at the time when John's service to God was very nearly complete; by definition (i.e. "to prepare the way"), John's ministry effectively ended when Jesus' ministry began (to understand this, and other examples, of someone preparing the way for someone else, see "Land On Which You Had Not Labored").
The Scriptures below, perhaps surprisingly, indicate that John, even at that late time, and despite earlier signs, was not yet fully certain of the identity of The Messiah that he was preparing the way for, even though he was absolutely certain that he was preparing the way for the Messiah (this is not unprecedented; there are numerous examples in the Bible of God's servants given only what they need to know, when they need to know it e.g. "I appeared to Abraham, to Isaac, and to Jacob, as God Almighty [i.e. El Shaddai, see El], but by my name The Lord [i.e. "Jehovah"] I did not make myself known to them" Exodus 6:3 RSV), but on the other hand there was most definitely no doubt in Jesus' mind that John was indeed the "Elijah to come" for that time (see The Elijahs). Jesus had a very high regard for John:
"And when Jesus had finished instructing His twelve disciples, He went on from there to teach and preach in their cities."
"Now when John heard in prison about the deeds of the Christ, he sent word by his disciples and said to Him, "Are you He who is to come, or shall we look for another?"
"And Jesus answered them, "Go and tell John what you hear and see: the blind receive their sight and the lame walk, lepers are cleansed and the deaf hear, and the dead are raised up, and the poor have good news preached to them. And blessed is he who takes no offense at Me."
"As they went away, Jesus began to speak to the crowds concerning John: "What did you go out into the wilderness to behold? A reed shaken by the wind? Why then did you go out? To see a man clothed in soft raiment? Behold, those who wear soft raiment are in kings' houses. Why then did you go out? To see a prophet? Yes, I tell you, and more than a prophet. This is he of whom it is written, 'Behold, I send my messenger before thy face, who shall prepare thy way before thee.'"
"Truly, I say to you, among those born of women there has risen no one greater than John the Baptist; yet he who is least in the kingdom of heaven is greater than he. From the days of John the Baptist until now the kingdom of heaven has suffered violence, and men of violence take it by force. For all the Prophets and the law prophesied until John; and if you are willing to accept it, he is Elijah who is to come." (Matthew 11:1-14 RSV)
Despite the very high regard, John's time was up.
"For Herod had seized John and bound him and put him in prison, for the sake of Herodias, his brother Philip's wife; because John said to him, "It is not lawful for you to have her." And though he wanted to put him to death, he feared the people, because they held him to be a prophet."
"But when Herod's birthday came, the daughter of Herodias danced before the company, and pleased Herod, so that he promised with an oath to give her whatever she might ask. Prompted by her mother, she said, "Give me the head of John the Baptist here on a platter." And the king was sorry; but because of his oaths and his guests he commanded it to be given; he sent and had John beheaded in the prison, and his head was brought on a platter and given to the girl, and she brought it to her mother. And his disciples came and took the body and buried it; and they went and told Jesus." (Matthew 14:3-12 RSV)
Fact Finder: How many rulers in the Herod dynasty are identified in Bible History?
See The Herods