It seems you have to be in the camp with some or all movement points. I took the time to promote but stayed in the camp and had the choice come up every turn. I even switched occupiers so my scout could get the xp from dispersing. And yes meanwhile another unit spawned I only did this because I had 3 units in good shape plus there were CS units around Anyway it actually seems quite possible, though not always easy, to exploit this. You can buy a warrior from the camp, then kill off the spear and take it, raid for some gold back, fight some more then clear it for 10xp to the unit of your choice. In the end I get a warrior, 2 or 3 era score a bunch of training and 10 free xp on a promoted unit for 1/2 the cost of a slinger. Deal! Double-edged sword though. Pedro incited a camp I was trying to farm against me just when the game decided to give the barbs swordsmen. Yeah I didn't wait for the official "Defeat" entry in my HoF lol.