The Crusades sponsored a constant flow of people between northern Europe and the Middle East. By 1150, the keeps and towers built in Europe were influenced by what knights and masons had seen in the Levant.
Byzantine towers, like the Black Tower of the Bosporus, were generally round. Byzantium had been involved in serious imperial wars for many centuries, and they knew the importance of a round wall. Round towers don’t provide corners to hide behind; they have good visibility from the top to everywhere. Square corners, by contrast, not only provide some defensive wall, but also create vulnerable places where sappers can dig under the corner and collapse it with fire. Even before gunpowder gave Europe real explosives, they were good at weakening stone buildings with extremely hot fires fueled by extra fat. (At least one castle was sapped with freshly-killed pigs placed into the tunnel next to the wood.) Round towers presented no obvious places to dig.
But round towers were much harder to plan as family residences. Byzantium built them only as forts and prisons; European lords intended to live there with wives and children. Square rooms are much easier to build, decorate and live in. You can put square rooms into a round tower, but it’s not easy, and it weakens the wall.
The Tour César in Provins, northern France, is a good example of how Europeans started to modify square keeps. It’s built on an octagonal base, so it wasn’t quite round, but neither was it presenting square corners. Where corners would have been, the flat octagonal walls had round towers. Round corner towers were often added to square buildings this way, since round towers were easy to design, always good for winding staircases.
They still built entrances well off the ground. Tour César now has a ground entrance, but in its day, it was reached 20 feet up by a wooden drawbridge. Additionally, it was built on a mound like earlier towers. Inside, it had two stories, both devoted to large vaulted halls. The upper one held dinners and feasts and was lit with a skylight, the lower was used for storage and other business. A side passage led to a nearby stair that ran down to a well. The whole keep was surrounded by a stone wall, called the curtain.
Other mid-12th century keeps were rounded off on all four corners so that all the outer walls were round, but inside the plan was still basically a square. When defense mattered more than livability, increasingly they went over to round towers. By 1200, most new towers were basically round.
During this time, the outer wall, the bailey, hadn’t changed much. A tower keep might have an inner wall, the curtain. This was the beginning of making it all more complicated. It was in the following centuries that things really got going.
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