I will camp against thee round about; i.e. "I will bring armed men against thee who shall encamp around the entire circuit of thy walls." There was small chance of forcing an entrance into Jerusalem on any side except the north; but, order to distress and harass her, an enemy with numerous forces would dispose them all round the walls, thus preventing all ingress or egress (see Luke 19:43).
And … lay siege against thee with a mount; or, with a mound. Artificial mounds were raised up against the walls of cities by the Assyrians, as a foundation from which to work their battering rams with greater advantage against the upper and weaker portion of the defenses.
And … raise forts against thee. "Forts" were usually movable, and accompanied the battering-ram for its better protection. Archers in the forts cleared the walls of their defenders, while the ram was employed in making a breach.