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human
[High Fantasy] What would defensive positions really look like instead of medieval castles?

Castles, keeps, and fortifications were an arms race against real-world weapons. Wood palisades fell out of favor to stone curtain walls, which then were bolstered and given skirts to stop siege weapons and digging under them, as well as a shift to rounded edges. Ultimately this led up to star forts that maximized kill zones and minimized weaknesses against more advanced artillery.

So, in a world with magic and dragons, what would they likely be like? Would they be more like modern bunkers designed to resist attacks from the air, more trenches, etc?
gpt
It would depend on the type of magic and dragon. For magic, devices similar to the construction of wands would be used to dampen their effects outside and within the walls tailored to frequencies beyond what's locally sanctioned. They'd have to placed these devices in a golden ratio throughout the castle not just for equal coverage throughout the area, but the magical defenses are strengthened through mathematical principles. 

This would make castles have a slight spiral flow towards the court from each outer wall. The magic dampening devices can be built into flying butresses to give magical and structural support. On the otherhand, fortresses built purely for magic defense would look more like rune-etched mirrors arranged in seemingly impossible arrangements like the staircases in the works of M.C. Escher serving to disorient any magician not familiar with its arcane construction. It'd be like trying to do math in your head while numbers are shouted at you left and right. These can normally be found in small form in the quarters of the court magician. Downside is that they aren't fit for defense against usual siege weapons.  

Dragons are difficult since the best defense a castle could have would be through the recycling of material from the corpses of the defeated. The hide can be used for the roofs of the more...politically essential areas of the kingdom while the walls of a castle can be built around the bones of the larger dragons giving a more arch-like appearance to conventional castles in our history. The more eclectic designs, in vulnerable areas, with the bones consist of multiple pyramids with hide-covered surfaces redirecting the blasts.