Captain can gain experience (should be a game option).
Each time the Captain wins a combat, he gains experience.
How it works.
It can be too difficult to make the Captain has secondary skills, and some of them are useless for the Captain (Ballistics, Diplomacy, Logistics, Mysticism, Navigation, Pathfinding, Scoutism, and new skills to come).
Instead, he can gain primary skills (Attack, Defense, Spell Power, Knowledge).
The Primary Skill Advancement is the same as for heroes types, so each time he win a combat he gains a Primary Skill point depending of his town type (Captain in Castle town, Captain in Wizard town, etc.).
Also to help the Captain (and Heroes) to defend the Town/Castle, some buildings gives him bonuses.
Each base creature dwellings, from level 1 to level 6, gives 1 point to Attack skill for defending Captain/Hero (temporary for the visiting hero defending the Town/Castle, permanently for the Captain as he cannot move outside the Town/Castle).
Ex: In a Wizard castle with Habitat (level 1), Pen (level 2), Foundry (level 3) already built, the defending Captain/Hero gains +3 in Attack during siege (temporary for the visiting hero).
The Left Turret, the Right Turret and the Moat buildings each gives 2 points to Defense skill for defending Captain/Hero (temporary for the visiting hero).
Each level of the Mage Guild gives 1 point to Spell Power and Knowledge for defending Captain/Hero (temporary for the visiting hero).
So, per example, a castle with 3 base creature dwellings, the Left Turret, the Moat and a Mage Guild level 3, gives +3 to Attack skill, + 4 to Defense skill, +3 to Spell Power, and +3 to Knowledge for the defending Captain/Hero (temporary for the visiting hero).
Note: The magic points given by temporary Knowledge are automatically assigned at the beginning of the combat, even if the hero does not spend the night in the castle (to get the extra magic points), and removed at the end of the turn if he has some points left (for the visiting hero).
The captain can also casts spells depending his Spell Power skill.
If the Captain has at least 6 in Spell Power, he can cast level 3 spells (if the Mage Guild is built and contains combat spells).
If the Captain has at least 8 in Spell Power, he can cast level 4 spells.
If the Captain has at least 10 in Spell Power, he can cast level 5 spells.
How about the possibility to make the Captain can equip artifacts?