it's not really just a skin, the Call of Olympus DLC quite radically changed a lot of things, and to be honest, at this point it's still pretty beta compared to the base game.

like, they added this "rally flag" that is cheaper than siccing an army on the frist nest of greed they run into, but you have to hang out with them, they stick with you, but you can also use the "armies" as well at the same time, so actually sometimes it's kinda boring because you can often use the two together once you upgrade your base to flatten every greedy portal until the end of the island, get the temple activated with the statue you get at teh other end of the map and voila... and they also boosted the ability of the monarch mounts to do stuff, like, there is this chimera, part lion, part dragon, it incinerates everything for about halfa screenfull as you jump forward, and they have a Cerberus that summons a free army that is undead ghosts, they aren't very strong though, the Chimera is better

but my point was just that they sorta are diverging from what the main theme of the game was - managing the objectives of NPCs who you command to do stuff to achieve game goals, they made it more of an action game which i think is going in the wrong direction, i played through near to the end of the Olympus game and it's not as fun