Mercenary Kings won't be available til 2014. The game had previously been shooting for a late 2013 launch.
According to the developer in its "As you've guessed it, Mercenary Kings will not be available for the 2013 calendar year. We're extremely grateful and excited about people asking for the game on PS4 or the completed version on Steam. It was our hope to have it completed and in your hands for the holidays but the game is not in the shape it needs to be for (its) official, full release.", which called the game "content-complete",
Mercenary Kings can best be called a Metal Slug-styled shoot 'em up in which players combat hordes of enemies while trying to rescue hostages. Players can upgrade and craft their armor and weaponry. The game also features four player co-op, both on- and offline.
The game has been announced for PC Steam and PlayStation 4 platforms, and has already seen a Steam Early Access release.