Gelimer retained 5,000-6,000 men under his own command while his brother Ammatus approached from the north with 6,000-7,000 troops, Ammatus scouting ahead of his troops in person. At Ad Decimum there was a narrow defile where the Byzantines could be trapped. The Byzantines did not know the layout of the road network and would probably be surprised if an army appeared behind them. When the Byzantines advanced towards Carthage they would most likely try to go through the path blocked by Gibamund who was ordered to charge them. This was supposed to push the Byzantines back into the valley and disorganise them. Gelimer would advance into the valley and attack them from behind.