The most important thing is that all units in the Castle are noticeably stronger than those of the same level in other factions. Halberdiers are the strongest of the first-tier units and have good movement. Archers attack twice. Griffins can respond to any attack. Crusaders attack twice. Monks have no penalties in melee combat. Champions have a cavalry bonus and are initially strong. Angels can resurrect, fly far, and boost morale.
It's a pity that the guild doesn't reach level 5, but at least there's Adela.