[City Unit Enchantment Bonus] +1 For Each Training Level Above 1 at Fortress, To Make Fortress Worth Using On High Essence Tiles.
So each City Unit Enchantment on a particular city that currently does +1 per essence (Aura of Grace, Aura of Might, Aura of Vitality, and Heart of Fire), Fortress Enchantments would add a +1 for each Training Level above 1 for each of these spells on the Fortress when the unit is trained. This would give a good reason to make a 2 or 3 base essence city into a Fortress, as if you're training level is 3 or 4, that's an extra +2 or +3 to the City Unit Enchantments which boost trained units. Rather than a high essence city ALWAYS being made a Conclave (boring), what not provide an interesting strategic choice to 1. encourage more building of Fortresses and 2. make it a harder choice between Fortress and Conclave for high essence cities?
PS: Also make sure that Henchmen and Sions either get their level-up choices (by levelling units from L1 upwards) or always starting them at Level 1. These units lose out a lot if they miss out on Trait Choices, and if a Fortress trains L4 units, these special units would never get a guaranteed path!