: : I imagine the troops must be coming from the lands down South.

: : Why then are they marching across Ithilien to reach Mordor.
: : Surely there must be an entrance to Mordor in the South or
: : South-East. Or else how would Sauron have moved his troops
: : before gaining control over Ithilien.

: There was a Nargil pass farther south that was dropped from the maps and story, but it may still have been there in Tolkien's conception. Sauron could have insisted on sending troops through Ithilien as a means of asserting his presence there.

If the Southrons had a cavalry contingent then leading them through Gorgoroth's wilderness would have been folly. Horses need fodder.