Part Sim, Part RTS and Part RPG-style exploration, Sky Citadel has you lead your own flying fortress in a world where the ground is a distant memory. There are a few floating islands around which people make their home, but they tend to be sparsely spread around the known world, making trade difficult. Most people live in flying ships of some kind: Some are new, built at great expense with the limited resources available while others are ancient relics, created in a more prosperous time, that have been recovered and recommissioned to meet demand.
You are one of the lucky few to have discovered a Sky Citadel, an entire floating city supported by elaborate engines and propellers: A true treasure of the ancient era. Fortunately, for your benefit, the citadel is also heavily fortified from the constant attacks you'll suffer as you defend your prize from jealous rivals. As well as maintaining the citadel, you can also upgrade it, build on it and house a large population of citizens looking for a home to call their own. The citadel also has various means to be self-sufficient, including a massive grappling hook for salvaging all sorts of items from the unseen depths.
This is pretty much how the game goes: Chase down rumours of treasure below the skyline or hunt after pirates that have gotten rich off the suffering of others, make money, expand on your citadel and continue to grow as a presence. Explore the world in your own way and collect any refugees that appear like they could be of some use to your operation. There'll be an overall plot involving an evil empire, with their own sky citadels, but the main plot won't be as obtrusive and railroad as in other games of this caliber. Get around to it whenever.
Just to go over the three game modes:
Overworld mode, which is where you'll move your citadel to the next destination, or look for treasure with the grappling hook.
Citadel mode, where you'll build all the additions, when they become available and when you have the resources/cash to build them. You can also configure the existing populance, assigning them quarters inside the citadel and jobs to perform (such as Repair or Engineering).
Battle mode, when you find yourself attacked by enemies. You can actually fight back in two ways: by assigning NPCs with a high accuracy score to the citadel's cannons and letting them take down the enemy as you move the citadel around for best coverage, or you can assign yourself to a cannon turret and take down the enemy personally in a FPS mode. There are also other ways to fight back, including magic and occasionally with the grappling hook, if you wanted to capture crippled enemy ships for their valuable resources.
There'll also be a captain's log for keeping up to date on your recent exploits, plus rumors and clues you find that will tell you where to head next. There's also a giant ancient map of the world that you'll be able to follow - and often need to edit as you discover changes made since the citadel was originally created.
