Fief was easy to take because there was no players to defend its, you can put as much pop and gold if there only 3 hired mercs this will be a mess anyway.
Only fief battles that were defended were when a faction hated another, AI fief should be defended by randomer but it should be encouraged (xp and gold ofc...).
In terms of pop, it's better to figure out a max pop for fief owned by a faction/clan, same for gold as faction will loot the gold of ai fief.
Too much gold to win in a loot will create a gap between fief faction and fief less faction.