Exactly what the title says.
Honestly, do you expect army bases to be completely defenseless? Or the National Guard in Washington DC not digging in when they hear aliens are coming?
In other words, there ought to be defensive works.
A couple examples:
Foxhole/Trench:Easily codable. It's a hole in the ground, for god's sake.
Sandbags: Not so easily codable. It ought to act as an obstacle to fire if someone's crouching. Also should add a TU penalty to jump over it.
Emplaced weapons: Just that. A stationary object that can combine with a soldier to make a soldier/weapon with zero mobility.