and how can you determine which one is bigger guild which one is smaller guild?
Tier 1 Castles would have the best rewards, so, bigger guilds would be interested in getting those castles and holding them for econs, low tier castles wouldn't have high-end rewards, but it'd be awesome to new players and guilds.
Of course, big guilds could attack low tier castles, but they'd leaving behind good rewards just for the sake of the ???, that said, their enemies would get their castle.