the invention of the cannon
Because Gunpowder made the walls obsolete as a means of defence.
Warriors are not, nor likely to be, obsolete.
Usually for a small time after they become obsolete.
8 track started to become obsolete in the early 1980's in the USA.
Sundial
yes
Floppy Discs
Stone castles have many small spaces between each other. As the space becomes bigger, the stone castles become weaker an weaker.
There are different reasons as to why castles became palatial. One main reason is because castles were no longer needed for defense, therefore were turned into palatial castles where people were free to visit and stay in.
Short answer: No. Long answer: Some forms of pencil will become obsolete, such as the #2 HB that has to be sharpened. People will just switch to #2 HB leads in mechanical pencils.
KKK became obsolete due to the, 1874 Alabama's Democratic Party that redeemed the state from Republican rule.
Artillery (Cannons) could collapse them.