Figure 1. Philon's system of ditches. [Marsden 91]
There are three types fortifications: field fortifications, continuous barriers and castles. Field fortifications were the simplest: "Originally such fortifications may have been derived from hunting, where various devices were used since prehistoric times to trap animals and to limit their movements..."[Van Creveld 25] Field fortifications were mostly temporary structures, and did not withstand attacks any better than one might expect of hasty works. Continuous barriers, such as Hadrian's Wall and the Great Wall of China were stronger but required far more resources. "It [required] managerial and organizational skills and resources, and the ability to put thousands and even tens of thousands of men to work in a purposeful, coordinated fashion while simultaneously keeping them fed, clothed, housed, and policed... It was a question of pouring sweat in order to save blood. "[Van Creveld 26-7] Niether of these types of fortification were effective against artillery, which led to the creation of the third type of fortification, the castle. Rather than trying to protect an entire region, it was far more reasonable to concentrate defenses on a single point. "However great the superiority that an invader enjoyed in the field, and however numerous his victories in battle, a country was not really occupied until its fortresses had been reduced."[Van Creveld 28]
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Figure 1. Philon's system of ditches. [Marsden 91] |
Catapults were one of the ways to reduce the fortress, more of a threat than mining since they could be effective from a long range. Marsden 90-1 quotes Philon offering advice for defending against catapults:
...it is essential to have at least three ditches in front of all fortifications. According to his measurements, the exterior lip of the outermost ditch must be at least 178 yards away from the city wall. "If ditches of this size and type have been dug, they will not be filled in quickly, and the one-talent stone-thrower, which is the most powerful, will either not get near the wall or, being deprived of support, will topple over." Probably a large stone-thrower had to stand within 150 yards of a wall to be effective, the machine being forced to shoot at a very low angle of elevation so that its shot would have the maximum possible forward momentum.
Catapults could be used by the defenders just as well as by the attackers. When castles and fortifications were built, catapults were often installed in the towers. During a siege the height of the towers in which the catapults were contained became a crucial factor. For example (assuming that all the catapults used in a siege were equal and had a range of 400 yards) an arrow fired from a forty foot tower perched on a fifty foot hill would land 450 yards from the tower. For the attacker to hit the top of said tower, their catapults would have to be a minimum of 120 yards closer. [Marsden 131-2] This extra height was not without price; it left a zone of dead ground at the base of the tower which the catapults could not aim into. From the 40 foot tower firing out over level ground, this dead zone was around 60 yards wide.[Marsden 118] Thus, putting additional catapults down near ground level could provide coverage for the dead zone. As Philon recommends,
Below [i.e. in front of] the curtain-walls and outworks, platforms are to be constructed for as many and as large engines as possible, some at surface level, others below ground level, so that there may be plenty of room for their operation, so that the detachments may not be hit, so that they may inflict casualties while themselves out of sight, and so that, when the enemy draws near, the aimers may not be handicapped by being unable to depress their engines.[Philon qtd Marsden 117]
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Figure 2. Fields of fire from a rectangular tower. [Marsden 142] |
Catapults as a defensive weapon have influenced the designs in which castles have been built, primarily in the shape of the towers that contained them. Two shapes are most conducive to catapults, rectangular towers and circular towers.
A rectangular tower offers three sides for a catapult to shoot from, giving three fields of fire that almost completely surround the tower. The angles of the fields comes from the width of the wall and the size of the window, as well as the distance of the catapult from the window. The catapult that is facing outward could be aligned with the center of the window, but the catapults at the side window were placed behind the window (towards the wall) since they had no need to fire behind the wall.[Marsden 139]
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Figure 3. Fields of fire from a semicircular tower. [Marsden 143] |
A semicircular tower could contain four windows, which extended the fields of fire from the rectangular tower from almost covering the field before to completely covering the field with some overlap. Again, the angles of the fields comes from the width of the wall and the size of the window, as well as the distance of the catapult from the window. The nature of positioning windows on a semicircular tower during construction, as opposed to the rectangular tower, was such that it afforded more flexibility. The windows could be placed so as to concentrate the fire, or spread out to provide maximum coverage. The curved surfaces also denied incoming projectiles a flat surface to impact, so that they would be more likely to just glance off with minimal damage.[Marsden 143]
From O'Connor.
Philon of Byzantium
Before describing the contents of Philon's masterpiece Mechanics let us give some small details of Philon's life which can be deduced from comments which he makes in this text. Certainly Philon describes journeys he had made to Rhodes and to Alexandria to study catapults. He appears to have discussed military applications of catapults with the rulers of Alexandria. The tone here would suggest that Philon was a wealthy man of independent means able to travel in the pursuit of his studies. On the other hand it is possible that he was considered the right sort of person whose advice should be sought on military matters and he may have been earned his living advising military rulers.
What exactly was in Philon's Mechanics treatise? We know that it had nine books:
...
This treatise is not just a work on what we would consider today to be applied mathematics. For example Book 8, in addition to describing ways of defending town walls from both land and sea attack, also stresses how important it is to have a good doctor available. Philon argues that those badly injured in attacks so that they cannot work again should be awarded pensions, and that the wives of those killed should be provided for.
To capture a town through a siege one must, according to Philon, make proper use of machines such as catapults and other war engines. In addition one must try to starve the inhabitants of the town, bribe suitable people to assist you, use his poison recipes to kill the inhabitants, and also use cryptography to pass secret messages. It would be interesting to have details of his proposed cryptography but unfortunately Philon's work on this topic has been lost.