Castles are odd, aint they?

[sushiboat]

I think the consistent thing to do would be to have researching mages who have to defend an unfortified province not be able to finish their turn of research. Then a fort allowing researchers to research uninterrupted would be a step up. I think that for a single unit to do both (research and defend) in one turn is unreasonable. Enemy attacks are disruptive, and forts are a buffer against that disruption -- not a way for one unit to do everything at once.