Ladders are one of the most ancient siege engines ever used by mankind. They were used in antiquity and the Middle Ages alike. This seems to be logical, because in every siege it was the aim of the attackers to get over some kind of fortification. Ladders could be used for this task and could be produced relatively fast. But, as with all things in life, there were also disadvantages. The use of a ladder in a siege was quite dangerous and the construction of a good siege ladder was not so easy as one might think.
The correct lenght of the ladder was very important. If it was too short, the attackers would not reach the top of the fortification. Was it too long it would collaps under the weight of the attacking soldiers and their armour. The length of the ladder had to suit the height of the wall. This was something one had to consider very carefully, especially if the target of the attack was surrounded by multiple walls with different heights. To learn the exact height of the wall two different methods existed. You could try to shoot a rope to the top of the wall so that it formed a triangle with the ground beneath it and the wall itself. Knowing the length of the rope and the distance between the point were you stood and the foot of the wall you could calculate the height of the wall. It was also possible to compare the shadow the wall threw at sunset with the shadow a 10-foot-long-pole threw at the same time. Now one could calculate the height of the wall. Both methods were not so easy as they may seem. Keep in mind that all actions had to be performed while the defenders shot everything they had at the engineers.
Not only normal ladders were used. There were also ladders that were mounted on wheels or that had a small wooden ramp on top, on which the attackers could reach the top of the wall.
As I mentioned before, attacking a fortification via a siege ladder was very dangerous. The attackers were only protected by their armour and shields. Even with their own archers trying to force the defenders into defilade the attackers were exposed to every kind of projectile the defenders had at their disposal. Defending archers, crossbowmen and in the late Middle Ages early firearms caused great casualties. Stones and even furniture was thrown at the attackers. Hot oil, boiling water and hot sand were gruesome weapons. All three substances found their way inside the armour and burned the attackers instantly. In some cases the defenders used beehives, which they threw down the walls.
Because of these dangers it was crucial to cover the distance between the foot of the ladder and the top of the wall very fast. As far as we know the warriors of the antiquity and the Middle Ages were very quick climbers. Soldiers were able to climb the ladder without using their arms. Therefore they could protect themselves with their shields. The knights were also very athletic warriors. They were able to climb a ladder in full plate armour by only pulling themselves up with their arms. But no matter how fast one could climb a ladder, he still had to face the defenders on the top of the wall who were all to eager to throw one back down.
Ladders are still in use in the modern military, for example to storm the upper floor of a building in combat. They therefore remain one of the oldest siege engines mankind has ever used.
Nossov, Konstantin. Ancient and Medieval Siege Weapons. A Fully Illustrated Guide to Siege Weapons and Tactics. Lyons Press Paperback edition 2012. Guilford, 2005.