Alright. I'm going to generalize this slightly, just to make it abit more fun.
There are aliens. Lots of them. They are here, and they will keep coming. We can't stop them at their source.
They aren't in our cities. We have time to set up defenses. We have unlimited resources.
That said, my first thought is to gather the entire population into dense areas. Central point of failure, yes, but central point of defense as well. We'll assume that we can sustain whatever society we decide to set up in these city walls.
Knowledge from past Alien movies:
They are intelligent. They are fast. They are strong.
They can be harmed by general munitions. Doing so may not be in your best interest (see: acid blood) but it is possible.
They can be harmed by extreme temperatures.
They can swim. They can jump, far. They can climb, extremely well.
Alright, we've got all the hoomans stuck in a big city. We need a wall. I would suggest a as-close-to-vertical, relatively slick wall. Possibly even slant it outwards, so they're climbing up a negative incline. See Diagram for example.
Code: Select all
That's an alien -> ..- \ <- That's a wall.
Personally, I think the buildings-on-stilts would be even better, but a lack of materials may make it really hard to do. (Not to mention, the aliens may get smart enough to start knocking out the stilts.)
If we're going way overboard, make a bowl out of said slick wall material, and pour enough dirt in to have sustainable agriculture. Aliens can't fly, and with the bowl, they can't dig their way in.
Now, as for defenses. Turrets would be awesome. We know they show up on motion scanners, so all we have to do is keep the turrets below the edge of the wall, on the outside, so if anything moves, they kill it. (Civilians don't get to go outside... no reason to, anyway.)
Snipers are a second line of defense.
... Boss aggro. I'll continue when I get home X.x
Now, as for the 'Airlock' design you posted up above (I think it really is more like an airlock than a DMZ.) It would be fairly effective for isolated instances, outside of the main city. Obviously, facehuggers aren't gonna get through that door alive (Unless they impregnate somebody on the outside, and if you're stupid enough to bring them back in after they're impregnated, it's your own damn fault.) As for soldiers, you better make sure you've got damn good doors.
This is the best defensive strategy I can come up with that seems fairly reasonable. (Well, except the bowl part, but that was just for fun.) The next step would probably be to just enclose the city-level areas with domes, but then you're almost at the level of making biospheres (With foot-thick steel walls, as opposed to tinfoil protecting you from the atmosphere... but hey.)
... Somebody wanna play the aliens, and figure out what's wrong with this defense so far?