This post is in category BUILDINGS
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The Normans of north-west France made improvements to castles, which they spread to England.
In the 12th century, the Templars from north-west France built in Portugal castles with projecting towers to give a line of sight to all approaches. The towers were made round to eliminate the vulnerability of square corners to rocks thrown by trebuchets. The design also included a tower called the castle keep.
The Templars built castles in Muslim lands during the Crusades, and Muslims copied these designs.
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early English castle
Muslims invented few, if any, of the defensive architecture features of the 12th century castles. Arrow slits were used in 214 BC in the city of Syracuse in Sicily. Moats were constructed in ancient Babylon. The Ishtar Gate of ancient Babylon contained a barbican. City walls of the Hittites and forts of the Roman Empire had crenulated parapets. http://downloads.bbc.co.uk/rmhttp/schools/primaryhistory/images/romans/defence_of_britain/r_porchseter_castle_wall.jpg
In Portugal, Romans built stone forts with high walls and strong towers. In the 8th century, Muslim Moors in Portugal built castles with decorative stonework and more heavily fortified the gates.
The Italian city of Squillace had originally been a Byzantine Christian fortress, and I presume the people of Squillace knew about Byzantine defensive castle architecture. Around the year 1040 (a half century before the First Crusade) the Muslim Saracens had been raiding the area. The people of Squillace asked for and received military help from the Normans of north-west France.
Soon afterwards Geoffrey II, Count of Anjou, built a castle in Squillace and at the same time also “ordered to build [in France] a more secure large stone castle”. http://www.travelvivi.com/10-most-astonishing-castles-in-france/
(round towers in photo were probably added later).
The Normans brought their castle design to England.
In the 12th century, the Templars from north-west France built in Portugal castles with projecting towers to give a line of sight to all approaches. The towers were made round to eliminate the vulnerability of square corners to rocks thrown by counterweight trebuchets. The design also included a tower called the castle keep. http://en.wikipedia.org/wiki/Convent_of_Christ_(Tomar)
During the Crusades, openings where an upper story floor meets the wall (called machicolations) and openings in the roof over a passageway (called murder holes) were used to drop things onto the attacker, but it is not known whether they were invented by Muslims or invented by Christians.
The Crusader castle at Tyre (modern day Lebanon) in 1187 had a ring of outer walls before the attackers could reach the castle. The castle included huge counterweight trebuchets mounted on massive tower.
Image by William M Connolley at en.wikipedia, via Wikimedia Commons.
image credit https://en.wikipedia.org/wiki/File:Castle-rising-castle.JPG