Here's the starting human base. To reach the aliens, head out the hall on the right.

The first hall. Dretches will usually rush in here at the start of a map. A couple turrets with a medistation/armory in back work great here.

The end of this hall terminates in a door. It's pretty standard for humans to stack 2 or more turrets immediately inside this door. This is about as far as humans can build forward at S1

Outside the door is a long hall with lots of ridges on the sides. At S2, humans can build forward along this hall, deconning the stuff in the previous hall.

Two more doors. The one on the right (the open one) leads to the aliens.

Which are under the floor of this scary-looking room.

The large gaps in the grating at the end of this room are where aliens (except tyrants) come up. Directly beneath the center grate is a light fixture that makes a great spot for a repeater.

Beneath the floor, looking at the door to the OM room. Aliens must run straight down a narrow passage between pipes and can be shot with relative ease. Ambitious builders can use the aforementioned light fixture repeater to plop a medistation and turret in this room. This pretty much seals the aliens' fate.

An excellent base location which given enough defence structures will often survive to the end of the game. Sometimes the OM is moved to an upper ledge. There are two incoming routes which are both easy to trap.

May be useful in an emergency. There are two incoming routes, the lower of which is hard to trap and needs an alien or two nearby.

One for the surprise factor - this is just a short corridor between two rooms and humans will not expect to find a base here. There are two incoming routes. The Overmind is fairly exposed here.

Another difficult base to defend. Though there is only one main incoming route, attackers can also drop grenades or jump down from the walkways above.