My alien base is as follows, without screenshots;
S1
Overmind: Default location
Acid tubes: Walled around OM, blocking ramp to OM, in recession near hallway entrance
Eggs: Two near OM, one at top of opposite ramp
Barricades: None
S2
Overmind: Default location
Acid tubes: Walls/ceiling around OM, walls/ceiling around main/hall entrances
Eggs: Ceiling around OM, top of ramp on ceiling (hallway side)
Barricades: None
Trappers: Ceiling recession near OM, top of OM-side ramp (wall), hallway entrance near floor (wall)
Boosters: Underneath hallway-side egg
S3
Hovels: Middle of OM-side ramp to block tardsuits and other humans without jetpacks/lucis
Hives: None
That's how I set up an alien ATCS base, and with adequate living defenders, it's fairly hard to kill without a massive spamsuit rush.