Jamieson, Fausset and Brown on Joshua 6:26

26. Joshua adjured them at that time--that is, imposed upon his countrymen a solemn oath, binding on themselves as well as their posterity, that they would never rebuild that city. Its destruction was designed by God to be a permanent memorial of His abhorrence of idolatry and its attendant vices.

Cursed be the man ... that riseth up and buildeth this city Jericho--that is, makes the daring attempt to build.

he shall lay the foundation thereof in his first-born, and in his youngest son shall he set up the gates of it--shall become childless--the first beginning being marked by the death of his oldest son, and his only surviving child dying at the time of its completion. This curse was accomplished five hundred fifty years after its denunciation (see on 1Ki 16:34).