outposts are created by armies, think about the Roman camp that evolved into so many cities

what about allowing armies to create them :

- all the units creating the outpost are tied to the outpost

- these units get a defense bonus

- these units can roam to attack enemies, but if they exit the ZOC the outpost decays and has to be rebuilt

- more units can be tied to the outpost and untied, to allow for units improvement

- if the units are beaten the winner has the choice of razing or keeping the outpost, the latter forcing him to tie units to the outpost

- after a while settlers can transform the outpost into a full grown city with some advantage (city level two for example after 50 turns)

- the outpost "costs" the upkeep of the units tied to it (because you can't move them)

- outposts should take several turns to be built to avoid "snaking" improved units (defense bonus)