traditional chariot warfare. The philosophically pacifist Mohists (followers of the philosopher Mozi) of the 5th century BC believed in aiding the defensive warfare of smaller Chinese states against the hostile offensive warfare of larger domineering states. The Mohists were renowned in the smaller states (and the enemies of the larger states) for the inventions of siege machinery to scale or destroy walls. These included traction trebuchet catapults, eight-foot-high ballistas, a wheeled siege ramp with grappling hooks known as the Cloud Bridge (the protractible, folded ramp slinging forward by means of a counterweight with rope and pulley), and wheeled 'hook-carts' used to latch large iron hooks onto the tops of walls to pull them down. When enemies attempted to dig tunnels under walls for mining or entry into the city, the defenders used large bellows (the type the Chinese commonly used in heating up a blast furnace for smelting cast iron) to pump smoke into the tunnels in order to suffocate the intruders. Advances in the prosecution of sieges in ancient and medieval times naturally encouraged the development of a variety of defensive countermeasures. In particular, medieval fortifications became progressively stronger—for example, the advent of the concentric castle from the period of the Crusades—and more dangerous to attackers—witness the increasing use of machicolations and murder-holes, as well the preparation of hot or incendiary substances. Arrowslits (also called arrow loops or loopholes), sally ports (airlock-like doors) for sallies and deep water wells were also integral means of resisting siege at this time. Particular attention would be paid to defending entrances, with gates protected by drawbridges, portcullises, and barbicans. Moats and other water defenses, whether natural or augmented, were also vital to defenders. In the European Middle Ages, virtually all large cities had city walls—Dubrovnik in Dalmatia is a well-preserved example—and more important cities had citadels, forts, or castles. Great effort was expended to ensure a good water supply inside the city in case of siege. In some cases, long tunnels were constructed to carry water into the city. Complex systems of tunnels were used for storage and communications in medieval cities like Tábor in Bohemia, similar to those used much later in Vietnam during the Vietnam War. Until the invention of gunpowder-based weapons (and the resulting higher-velocity projectiles), the balance of power and logistics definitely favored the defender. With the invention of gunpowder, cannon and mortars and howitzers (in modern times), the