Sky Fortress

For gameplay purposes, Sky Fortresses function much like Structures, with the key difference that fortresses are gifted with 4-axis mobility, and can even levitate above the landscape! Well, sort of.

The caveat is that the Sky Fortress Turbines which generate the levitation field-thingy which keeps the fortress suspended in the... well, sky, can only safely move a fortress at an absolutely glacial rate of 1 metre per second. Additionally, whether the fortress is moving or not, these turbines require a constant supply of Power to maintain the levitation field.

The turbines have a kind of internal energy storage that allows them to maintain the field for some time in the case of a power failure, but once that runs out, the field becomes unstable, and will begin to tear the fortress apart at the seams!

Oh, and lastly, the further a fortress rises or descends above or below sea level, the more power the turbines will require to maintain the levitation field. This means that fortresses are economically kneecapped in terms of their operational minimum and maximum altitudes.

Why even use fortresses? Why not Structures?

1. Structures are fixed to the terrain, meaning that if a structure is spawned in the middle of the ocean, it will immediately attach itself to the seabed.
2. With the exception of those in the AotE map, many of the resource deposits are located on the seafloor. For example, in Neter's default designer mode, the deposit adjacent to the spawn point sits at an average depth of 247m.
3. Resource Gatherers need to be above water in order to function.
See the problem?

Now, one could of course build a structure that's tall enough to breach the ocean surface, and then mount the Resource Gatherers up there, but that can be a bit cumbersome as the terrain height/ocean depth varies across the map, and unless you customize each offshore resource-gathering structure to match the depth of water at its location, these structures will be too short to reach the surface at the sites of some deposits, and will rise above the seas to become ugly eyesores elsewhere.

Meanwhile, fortresses simply hover at a set altitude. They require a fairly steady supply of power to do it, but that's the cost of convenience. Furthermore, as fortresses are capable of basic movement, they can at least attempt to imitate vehicular combat behaviors, such as pointing their "front" towards a target, which means that they can, at least in principle, make use of heavy non-turreted projectile and hitscan weapons.