ship types with King Helü of Wu (r. 514 BC – 496 BC) while explaining military preparedness. Before labeling the types of warships used, Zixu said: Medieval and later use With the collapse of the Western Roman Empire into independent states, and the Eastern Roman Empire on the defensive, the use of siege towers reached its height during the medieval period. Siege towers were used when the Avars laid siege unsuccessfully to Constantinople in 626, as the Chronicon Paschale recounts: At this siege, the attackers also made use mobile armoured shelters known as sows or cats, which were used throughout the medieval period and allowed workers to fill in moats with protection from the defenders (thus levelling the ground for the siege towers to be moved to the walls). However, the construction of a sloping talus at the base of a castle wall (as was common in crusader fortification) could have reduced the effectiveness of this tactic to an extent. Siege towers also became more elaborate during the medieval period; at the Siege of Kenilworth in 1266, for example, 200 archers and 11 catapults operated from a single tower. Even then, the siege lasted almost a year, making it the longest siege in all of English history. They were not invulnerable either, as during the Fall of Constantinople in 1453, Ottoman siege towers were sprayed by the defenders with Greek fire. Siege towers became vulnerable and obsolete with the development of large cannon. They had only ever existed to get assaulting troops over high walls and towers and large cannons also made high walls obsolete as fortification took a new direction. However, later constructions known as 'battery towers took on a similar role in the gunpowder age; like siege-towers, these were built out of wood on-site for mounting siege artillery. One of these was built by the Russian military engineer Ivan Vyrodkov during the Siege of Kazan in 1552 (as part of the Russo-Kazan Wars), and could hold ten large-calibre cannon and fifty lighter cannons. Likely, it was a development of the gulyay-gorod (that is a mobile fortification assembled on wagons or sleds from prefabricated wall-sized shields with holes for cannons). Later battery towers were often used by the Ukrainian Cossacks. Modern parallels In modern warfare, some vehicles used by police tactical units, counterterrorists, and special forces can be fitted with mechanical assault ladders with ramps. These are essentially modern