The fortress was the
epitomy of
castle technology. The castle
era began with the
fort, which evolved into a
keep, then a castle, then a
citadel and finally a fortress. The fortress was
carefully planned, constructed over a very long time and was a direct
response to
gunpowder technology. It was only used during the very late
Late Medieval Period, which began in 1400 A.D., and was quickly
obsolete with the end of the castle age, but while it
survived it was incredibly
formidable.
The fortress began with a towering outer wall, which was almost as high as the fortress itself, and at equal intervals were placed culverin (early cannons) towers. The outer walls were reinforced with steel and were several inches thick. Due to the height, it meant a very high angle of fire was required to actually hit the fortress itself, and this was not always easy, and often not possible, with early cannons. Therefore, they were generally forced to fire at the walls themselves, which was the plan of the fortress design. These walls were several inches thick of thouroghly reinforced concrete (think of a dam wall, but keep in mind its still early engineering), and could take hit after hit. Gunpowder was still a new technology, and cannon balls were fairly small, non-explosive things at this stage, and thus the high impact was absorbed. Cannons were also problematic, sometimes exploding on themselves and failing, and there was not a constant supply of gunpowder, and therefore a constant bombardment was more or less impossible.
Besiegers were thus forced to bring outdated siege weapons to the field in order to keep the pace on the bombardment, but this proved ineffective. Even the most fearsome of the siege weapons, the Trebuchet, could not stand up to reinforced concrete, and the walls simply absorbed the impact with virtually no damage. All this while, the besiegers are being besieged themselves, as the fortress is outfitted with culverin towers. Culverins had the most range out of any siege weapon, and therefore, if the besiegers were within range to use their culverins, the besieged could return fire. This meant that the besiegers were always in a terrible position when assaulting a fortress, and not with an extreme upper hand as it had been with earlier versions of the castle.
If the besiegers decided to sit outside the range of culverin towers and starve them out, they had a very long wait. Fortresses were immensely large, and could hold over a thousand men. Even at full capacity, they were constantly stocked with food that could last up to five years, and even then there were other ways to get food in. During the High Medieval Period, which began in 1200, castles began to incorporate back doors and tunnels to allow food to be snuck in. The fortress had several tunnels leading elsewhere, so that if one was discovered, another could be used to being in food. Therefore, a besieged fortress could last for fifteen years and quite conceivably more.
Therefore, the only other option was a direct assault, and this was costly. The outer wall was practically vertical, and so tall that no ladder or grappling hook could reach its peak. It was thus impossible to scale, forcing the attackers to take down the only entrance which was, of course, the gate. Fortresses usually had a moat and a drawbridge, however, and so the attackers were forced to dive into the moat, which was often quite deep and therefore hard to swim in with armour, and then had to force open the drawbridge. All the while, they had boiling oil, dead bodies, flaming arrows and of course, guns, bombarding them from above. When they finally got the drawbridge down they had already lost many men, and they now had to bring a battering ram up to the porticulis, as they were bombarded with more oil, and arrows from murder holes.
If and when they broke down the porticulis, they were now met with the first inner ring wall. The defenders now fell back to this wall via overhead walkways, and were therefore not exposed to the attackers. The same drill as above happened here, and was repeated for the next three inner ring walls. Therefore, the attackers were forced through a five line defense (this five line defense is a counter adaptation of Caesar's siege works) and if the last ring wall was breached, the defenders abandoned their positions and retreated into the fortress.
The fortress itself was more like a citadel, within a five line defense, and this made it a fortress. It was more or less another defense on its own. Within the inner wall lay a small part of the town (though quite large in comparison to earlier castles), with not only markets but some residential housing and military buildings. There was then often a second moat and drawbridge, and as the defenders retreated into the citadel, they took defensive positions again. There was quite a large open space between the last inner ring wall and the citadel, but it was fairly close quarters, and so culverins and other cannons were now useless. Unlike earlier castles, however, siege weapons such as ballistae were still quite effective, and so the citadel was outfitted with ballistae.
So the attackers were still being bombarded, and the citadel walls were reinforced like the outer walls. If the attackers decided to retreat, and the defenders believed they were strong enough, they would break the siege and engage them in the small town area. In order to fully defeat the defenders, the attackers had to, again, force open the drawbridge, break down the porticulis, endure murder holes, then, and only then, could they secure victory by capturing or defeating the defenders inside the citadel. By this stage, however, thousands upon thousands would have died, the defenders would have had very limited casualties up until this point, and the defenders' morale would be high compared to the attackers, so the defenders still had a chance of victory here. Therefore, the fortress was the ultimate defense in the Medieval Age.