Ok so everyone reckons using a fort to house merchants on the same resource is an exploit. Putting a stack into a military unit over a resource is not. Ill explain.

I noticed that using a small leftover unit placed (Like upkeep 5 fl)on a resource I could get the same effect(as a fort). But the important thing I noticed is that the merchant says something like " thank you for your protection lord" when joined to that unit, that means that it is an intentional programming thingo not an oversight or bug, therefore it seems to me a legitimate strategy to use.

It would be certainly cool if the ai did likewise with its low power merchants, then the human player nearby could spy the unit and attack it leaving a stack of unprotected merchants to acquire!. ( Or vice-versa ai attacks my merchant stack)

The other advantage is that you can move this stack of merchants as one to another resource.