Jamieson, Fausset and Brown on 2 Kings 25:4

4. the city was broken up--that is, a breach was effected, as we are elsewhere informed, in a part of the wall belonging to the lower city (2Ch 32:5; 33:14).

the men of war fled by night by the way of the gate between two walls, which is by the king's garden--The king's garden was (Ne 3:15) at the pool of Siloam, that is, at the mouth of the Tyropæon. A trace of the outermost of these walls appears to be still extant in the rude pathway which crosses the mouth of the Tyropæon, on a mound hard by the old mulberry tree, which marks the traditional spot of Isaiah's martyrdom [Robinson]. It is probable that the besiegers had overlooked this pass.

the king went ... toward the plain--that is, the Ghor, or valley of Jordan, estimated at five hours' distance from Jerusalem. The plain near Jericho is about eleven or twelve miles broad.