Back then I could only experience it on a pirate CD, with cut videos, german voiceover and MTL'd text. Had no understanding what happens in the plot or why missions had me do this or that. But still I fell in love with the atmosphere and the idea of it.
So, what was so cool about it? An action/adventure game with layers of gameplay: you had your character (it could be the girl in screenshots or one of two other characters) to run around floating islands, build/repair stuff and do infiltrations on foot. and you had your flying island "ship" where you had to manage build space, energy, turret/hangar placements and such. Hangars produced planes that you could let go in auto mode or you could pilot them yourself while island's guns fended off attackers.
Missions were fairly linear but choosing a different character to play as changed the form and function of some buildings, most importantly turrets, energy plants and planes. Choice was between Fairy Girl (in screenshots) having smaller, harder to hit buildings with medium tracking machine guns; Brute Guy with slow shooting cannons and diesel-punk (smoke and grime) aesthetics; and Tinker Guy whose turrets and powerplants were big, very easy to hit but boasted best tracking and medium damage output.