They did not change from shell keep castles to square keep castle, they existed at the same time.
Most shell keeps were a result of replacing the wooden towers on top of the man made Mottes (erected as a quick way of making a castle) with stone buildings, as they were man made it was often thought they would not take the weight of keep with very tall thick walls like the Tower of London so they made shell keeps, with thinner walls.
To build a substantial keep they would have to demolish the Motte. You would have thicker stronger walls with a square tower, but the extra height on top of the motte is also a defensive advantage, and it woule be cheaper.
1981
Because the cannon was invented therefore blew castles up :)
Because he had built strong secure castles and he had enough of them around the contry and he was happy with their design which was a pointed tower castle.
because they where to old and there was no longer any materials to build them hope this helps
Because Chuck Norris Started in the movie business
Castle building stopped when they were no longer able to be defended. In the 15 century, artillery such as cannon balls and mortar were able to break through the stone walls.
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In addition to losing their effectiveness as defensive structures with advances in technology, as civil strife within the country declined, so did the necessity of such structures.
because they thought that if they lived in castles they would get eaten by the dragon ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
castle buliding came to an ending because of the gunpowder becoming a weapon and stone can not defeat gunpowder so alternate mean of defeat were sought
it was good because they could be guarded non- stop. so no prisinors could get out and they could only get in when a security guard put them in
The introduction of gunpowder made castles inefficient.