Aliens Sentry Guns

To create a chokepoint - one narrow means of access that if breached would require them to escape as is what eventually happens.

Think 4 player command and conquer, rather than deal with your enemies coming from one of three directions you heavily defend and block two of the three accessways to your base, thus leaving one path into your base, which if narrow enough can be easily defended with little effort!

Thats exactly correct a chokepoint in any escape gives you more time to lay waste to anything that has to try and clammer and hide but no-where to run.

I think the point is that Hicks and Ripley thought that they could eventually get through the pressure door.

What Anunnaki50 said....

I think if they ever came to the only escape being that door they would want those guns to kill off what ever may be waiting behind it for when ever it gets opened.