Escalade is the act of scaling defensive walls or ramparts with the aid of ladders, and was a prominent feature of sieges in ancient and medieval warfare. It was one of the most direct options available for attacking a fortification, but was also one of the most dangerous.
Escalade consisted simply of soldiers advancing to the base of a wall, setting ladders, and climbing to engage the defending forces. This would generally be conducted in the face of arrow fire from the battlements, and the defenders would naturally attempt to push ladders away from the wall. Heated or incendiary substances such as boiling water, heated sand, and pitch-coated missiles were sometimes poured on attacking soldiers. As the result of all this, it was often difficult for attackers to reach the top of the wall. Those that did so could initially be heavily outnumbered by the defenders on the walls, and could only push into the defenses after suffering heavy attrition.
Fortifications were often constructed in such a way as to impede escalade, or at least to make it a less attractive option. Some of the measures taken to counter escalade included the digging of moats (which prevented ladder-bearing soldiers from reaching the base of a wall), the construction of machicolations (which facilitated attacks on enemy soldiers while they climbed), and walls incorporating a talus feature.
Because of the difficulties involved, escalade was usually very costly for the attackers.