Easily the best F2P MMORPG there is.
It uses a unique turn-based strategy battle system with optional RTS elements, with a multitude of mercenaries that aid you in battle. In Atlantica Online, no one person has the same formation and strategy.
As you progress, you will be trading mercenaries out and trying new strategies. Would a Cannoneer work better than an Artilleryman? Would you prefer the Druid or the Champion? Or perhaps you'd like to get an ultimate, yet extremely expensive formation composing of all Class-B and Class-A mercenaries?
The quests are quite good also. The immersive storyline revolving around the cruel civilization of Atlantis and the corrupting mineral Oriharukon is unique all in it's own. You will come across thousands of monsters, each with 4 tiers of info to collect.
This game is different from most MMORPGs, and to some it may seem to different from the norm, but to those looking for something fresh, new, and free, Atlantica is your best bet.