You can find 38 of my airships here:
26 of my buildings here:
They are designed for the "Conquest" mode, meaning I was trying to minimize their cost and maintenance cost. Without making them visually unappealing or stupid though...
For the minor settlements I am just using a big bunch of zero-maintenance-cost buildings.
With defensive approach AI can not do anything to you even on the highest difficulty. And when he builds ridiculous amount of buildings himself, you just smash them with Siege Machine.
Siege Machine I
Heavy Metal armored Airship with Torpedoes. Remarkably good against settlements fully covered by rocket defences.
It does not require any Tech (0 Tier) and costs less than 200$.
0 Tier Bomber.
Heavy Caliber II
Advanced heavy ship to make some holes. Can not reach very high though.
One of my basic ships to blast them with rockets.
The Fist III
With that you can ram and board them.
Marines are just landing and going for a war.
Armed with machineguns to whipe that small aircraft and infantry out.
Multi-directional small defence III
Zero maintenance cost building which defends from all sides.
Zero maintenance cost building equipped with machineguns on both sides. Not the most practical, but looks cool. And sometimes makes AI astonished.
0 Tier building. Zero maintenance. I even used grenades on that. Not effective, but cheap.
Frontal fortification III
Defends from the front, has weak defence backwards.
Frontal fortification II
Cheaper version has no defence backwards whatsoever.
Anti-aircraft system III
Mostly against the airships above, but can deal with side defences too.
Quite versatile multi-directional defence.
Launches small aircraft. That is all it have to do.