Cassivellaunus realised he could not defeat Caesar in a pitched battle. Disbanding the majority of his force and relying on the mobility of his 4,000 chariots and superior knowledge of the terrain, he used guerrilla tactics to slow the Roman advance. By the time Caesar reached the Thames, the one fordable place available to him had been fortified with sharpened stakes, both on the shore and under the water, and the far bank was defended. Second Century sources state that Caesar used a large war elephant, which was equipped with armour and carried archers and slingers in its tower, to put the defenders to flight. When this unknown creature entered the river, the Britons and their horses fled and the Roman army crossed over and entered Cassivellaunus' territory.