Jamieson, Fausset and Brown on Jeremiah 39:4

4. the king's garden--The "gate" to it from the upper, city above was appropriated to the kings alone; stairs" led down from Mount Zion and the palace to the king's garden below (Ne 3:15).

two walls--Zedekiah might have held the upper city longer, but want of provisions drove him to flee by the double wall south of Zion, towards the plains of Jericho (Jr 39:5), in order to escape beyond Jordan to Arabia-Deserta. He broke an opening in the wall to get out (Eze 12:12).