Genesis 39:20 And Joseph's master took him, and put him into the prison, a place where the king's prisoners were bound: and he was there in the prison. - Free Bible Online

      20. Joseph's master took him, and put him into the prison--the roundhouse, from the form of its construction, usually attached to the dwelling of such an officer as Potiphar. It was partly a subterranean dungeon (Ge 41:14), though the brick-built walls rose considerably above the surface of the ground, and were surmounted by a vaulted roof somewhat in the form of an inverted bowl. Into such a dungeon Potiphar, in the first ebullition of rage, threw Joseph and ordered him to be subjected further to as great harshness of treatment (Ps 105:18) as he dared; for the power of masters over their slaves was very properly restrained by law, and the murder of a slave was a capital crime.
      a place where the king's prisoners were bound--Though prisons seem to have been an inseparable appendage of the palaces, this was not a common jail--it was the receptacle of state criminals; and, therefore, it may be presumed that more than ordinary strictness and vigilance were exercised over the prisoners. In general, however, the Egyptian, like other Oriental prisons, were used solely for the purposes of detention. Accused persons were cast into them until the charges against them could be investigated; and though the jailer was responsible for the appearance of those placed under his custody, yet, provided they were produced when called, he was never interrogated as to the way in which he had kept them.

JFB.