[FreeFull]

Before you arm the thing you also have to make sure it's tilted just right, because of the tilt switch, which I imagine is rather tricky.

[raisedbyninjas]

It was made with threaded leveling feet and bubble levels. According to Wikipedia, bubble levels can be made with quite a bit of precision.

[jamie_ca]

Yeah, the write-ups I've read all said that it was very precisely levelled before arming.