Castles! What about castles in western Europe? According to Van Dale dictionary a 'castle' (dutch 'kasteel') is: 1 reinforced building 2 large and beautiful house. In a nutshell it tells it all. One thing missing: it's historical development.
Phase 1. Safe home in times of endemic wars
Starting in the 9th century feudal lords built private fortress-residences for defence. Wars were endemic in those times. The fortress was built on a defensible height. Surrounded by a dry ditch and surrounded by a wooden palisade.
Phase 2. Siegproof
Castles and outer walls were made out of stone. Walls get thicker. Sophisticated circuits of curving outer walls and towers to defend all sections of the wall. Never ending arms race of siegers and defenders.
Phase 3. Military importance lost AND residence of landowner
With the use of gunpowder and artillery the castle lost its military importance around the 15th century. The thick stone walls no longer withstand bombardments with cannonballs. New and bigger windows. Gardens. House of a rich landowner of noble origin.
Phase 4. Make it profitable
Taxes on inheritance and costs of materials and personnel make most castles too expensive to live in since the 20th century. Castles no longer belong to the same family who lived there for centuries (generation after generation). 3 D's that causes selling castles: death, debts and divorce. Castles have to be profitable. Selling home made country products. Museum. Zoo. Country garden. Bed and breakfast. Restaurant. Hotel.