Well said Hoel,

Castles changed shape considerably over their period of existence depending on the weapons and technology available at the time but we tend to focus on castles where the crossbow and siege engines were the best that could be had. As soon as powder came in they all changed shape.

I can add just one or two more points. Bastions were at corners and along the flat parts of a wall and were used to pack troops into so that they could fire parallel to the adjoining wall. When ladders and assault towers were placed against the wall then you could get some fire onto them without leaning over the top of the wall and getting shot up.

Also, whilst a round outer wall is efficient money wise, a castles main weakness is through a siege where a curtain of men prevented food and supplies getting in. A ring is also the most efficient shape for them too. So in latter periods some castles had spiky bits of fortifications protruding from them so that the circle of men one bowshot distance from the castle had to be that much bigger and therefore harder to maintain and costly to siege. Dover castle has an excellent example.

http://maps.google.co.uk/maps?hl=en&...06545&t=h&z=18

Another thing which you touched on is about moats. Water filled moats were employed mainly to suppress sappers. These were chaps which burrowed under the outer walls and tried to destabilize them into falling down or in rarer cases burrow a tunnel fully under. Thats why most curtains and towers have wedge shaped bottoms to them. The water filled moat serves to drown sappers so that they cant start a tunnel.

The whole topic goes on and on tho eh ? Its the to-ing and fro-ing of the arms race.