Cannon Soldiers look like other enemy Soldiers, wearing horned blue armor and manning a relatively small red cannon. They do not move from their position, but they do turn to aim at Link, and return to their spot after being knocked back slightly by the recoiling cannon. Their cannon fires spiked cannonballs. They only do half a heart of damage and despite having blue armor, they only take two attacks from the Fighter's Sword to defeat them, rather than three.
As they use the same graphic tiles as the Ball and Chain Trooper it is likely that they would have appeared within Hyrule Castle or the Hyrule Castle Tower, and would have been placed in a hard to reach location where they can fire upon Link from one or multiple compass points without fear of retaliation.