You will need this:
mora_order5 requires factions { greek_cities, } and building_present_min_level hinterland_settlement_determination fortified_city and building_present_min_level legion_suppy legion_suppy and hidden_resource gre
Which is the fifth level of Spartan barracks, in a Fortified city, with the army rations supply building, and the city must have the hidden resource 'gre', meaning Greek.
That means that only certain cities will allow you to build this barracks.
There are currently only two places (as I recall), where Epic stone walls can be built, Syracuse and Rome. One reason is simply historical.....these two cities had very extensive walls. The second is 'practical'....the Epic stone walls are bugged (too high for the ladders), and the animations for the gates are horrid. In RSIII, these walls will be removed entirely. Settlements will only be able to build the 'Stone' walls, except for Syracuse and Rome, which will be allowed the Large stone walls.