A commander could prevent being flanked by anchoring one or both parts of his line on terrain impassable to his enemies, such as gorges, lakes or mountains. Notable examples of this being the Spartans at the Battle of Thermopylae, Hannibal at the Battle of Lake Trasimene, and the Romans at the Battle of Watling Street. Although not strictly impassable, woods, forests, rivers, uneven, and marshy ground can also be used to anchor a flank: Henry V at Agincourt. However, in such instances, it was still wise to have skirmishers covering the flanks.