This is because the price is based on realistic market prices and actual sales
Captain N'ghathrod Captain N'ghathrod cares about horrors, and it wants you to actively attack with horror creatures
Marik and Kaiba each wanted to duel Yugi so they could get Slifer, while Joey wanted to duel Yugi in the finals and didn't want Yugi to have to duel Marik, so he used a strategy to make both him and Marik run out of lifepoints pretty quickly
Because you can get cards that have defender without being Walls, the two are not synonymous
Alpha, Beta, Unlimited, and other early sets are especially important because their cards were printed in low quantities and are highly collectible
Any character of the appropriate cost can sing them, regardless of what set or color they are from, as long as they are in your deck legally