The methods of construction seen in 13th-century Crusader castles differed little from those of the previous century. Meanwhile, documentary sources shed an interesting light on the men who actually designed or supervised the construction of Crusader fortifications - the engigneors. These men were not merely military engineers but were often multi-talented individuals with numerous different skills. They could have high status, though not being members of the aristocratic elite, and appear to have been recruited from various ethnic groups including Greeks, Armenians and Jews and Western Europeans.
Other specialists included men whose task was to feed and clothe the people on the building site. In addition to those involved in building work, soldiers defended the place and the resulting numbers could run into the tens of thousands. Perhaps because some projects were now so huge, these numbers had increased during the early 13th century, resulting in the effective conscription of some local populations who were then organised into what was almost an army of militarised artisan-soldiers. Prisoners of war could also be used as slave labour, but they required close supervision and were not entrusted with responsible tasks.
Details of construction techniques inevitably differed according to the nature of the terrain, and archaeological excavations show some unusual variations. For example, when the Crusaders refortified Ascalon they built thick walls with narrow courses of dressed ashlar around a poured concrete core with through columns. These were raised on a sloping artificial mound, largely of sand, and rising to a height of up to 10m with horizontal offsets at approximately one- to two-metre intervals. The mound was then lined with stone to form a glacis.
At Arsuf, the town's circuit wall was again built on sand foundations. The reasons are not entirely clear, but may have helped the structure absorb earthquakes, which are common in this region, and allowed water to drain beneath the walls. This was an architectural idea which long pre-dated the arrival of the Crusaders, yet it caused problems for modern archaeologists. For example, while the sand foundations at Arsuf were being studied, a family of foxes decided that they were an ideal place to excavate a home, causing the medieval wall to crack. Efforts to drive away the foxes failed, until the placing of lion's faeces from a helpful zoo finally convinced the foxes that they were no longer 'top predators', whereupon they moved elsewhere. Whether the engineers of medieval Arsuf and Ascalon faced similar problems is not recorded.
Given the variety of very localised problems, one would expect Crusader castle builders to have employed local masons with local experience. Yet the Military Orders that built castles in Cilicia rarely relied on Armenian masons. On the other hand the Crusaders used ancient or early medieval building material where this was available. When constructing the Sea Castle at Sidon they apparently hauled such masonry from the shallow sea.
So many variations were involved in these buildings that it is difficult to distinguish Crusader and Islamic work. However, one idea that does seem to have spread from east to west was the use of embossed masonry. This had been known in the Middle East since ancient times but did not appear in western Europe - with the possible exception of Alsace - before the 13th century. It offered additional protection against missiles, since the bosses ensured that mangonel balls rarely struck a wall square but normally hit a glancing blow. Their appearance at Acre and Tyre at the start of the 13th century might be further evidence for the increasing importance of large trebuchets in siege warfare.
Another distinctive feature in the Sea Castle at Sidon was the use of some dry-stone construction, presumably because the available mortar would not set properly when in contact with seawater. There were continued references to iron cramps or pins being used to strengthen Crusader fortifications, sometimes set into lead, though the iron at Sidon seems to have been driven into wood. More wood was, in fact, used in the construction of Crusader castles than is generally realised. Many roofs were of timber, especially for outbuildings such as stables. Large amounts of timber were similarly used for scaffolding.
This carved marble slab was found in the ruins of Ascalon. It consists of a fine Arabic inscription made during the pre-Crusader Fatimid period, to which the repeated coat-of-arms of Sir Hugh Wake of Lincolnshire were crudely added in the mid 13th century.
The Tower of Flies once guarded the entrance to the outer harbour of Acre. A mole may originally have enclosed the southern side of this harbour, extending from the base of the tower to the seaward end of the inner eastern wall of the city.
The well-known manuscript called De constructione castri Saphet was made for Armand du Perigord, Grand Master of the Templars, and is dated 1264. It provides details about the Crusaders' attempts to refortify Safad in the 1240s, but was previously regarded as controversial because it apparently did not fit the evidence of the site. For example it mentioned an internal rampart (in muris), and a foss (fossatis) within an external wall (antemuralia) which itself had a moat (scama) and seven towers. Also the garrison appeared impossibly numerous. In fact the Mamluks besieged Safad in 1266 and when the castle capitulated the victors reportedly found a garrison of 4,000 soldiers. However, more recent archaeological work shows that the document of 1264 was much closer to reality than expected.
Even more detailed information about 13th-century construction techniques was uncovered at the so-called Courthouse site in Acre, which exposed part of the Crusader city's outer wall, a tower and part of the moat with a masonry counterscarp. The tower was made of ashlar, pieces of which featured traces of plaster, suggesting they had previously been used elsewhere. The outer ashlar was finely finished and was laid in mortar 3cm thick. But the ashlar stones of the inner face were not so well dressed, with smaller stones being added to level the courses or fill gaps. This inner face was then plastered. The core of the wall was almost 2m thick, consisting of concrete into which rough 'field' stones and dressed stones were irregularly set. Evidence from other Crusader sites indicates that such inner cores were not just dumped inside the outer facings. In many cases they were carefully made and proved just as strong as the regular facings. The city wall of Acre, as exposed at the Courthouse site, was about 3m thick at its base, tapering slightly as it rose. It was laid upon bedrock, which is today not only below the water table but below the current sea level. However, the sea level has changed since the 13th century. The lower three courses were of large dressed stones, and a vertical seam, which does not continue in the higher courses, shows that the planners decided to enlarge the tower after work had already begun. The bottom of the moat corresponded to the third course of stones, and above these foundations the upper part of the wall was built of smooth ashlar.