Might not be 'cheap' but it might be possible with bits of recycled code.
Target spotters, sensors, and range finders.
Detectors: motion tracker/tripwire/pressure sensor/other detects something abnormal, and relays the alert to other systems on the same power grid. An alert pops up on a monitor in your base, you get an alert, (optional, enabled by default) and you can then either send a team out to investigate, gather more data, or start shooting with a mortar or ten.
Cameras track the movement of targets in real time, allowing you to hit mortar shots much more consistently.
Flares guarantee a mortar hit on a nearby target, but blind nearby pawns.
Etc. basically, make playing defensively inside and outside more interactive. Oh, you're perfectly safe in your mountain, but blind to the outside world. Want to look around? Gotta run some cables out there. You on the other hand are worse for wear but you can see threats coming much more easily. Stuff like that.
Target spotters, sensors, and range finders.
Detectors: motion tracker/tripwire/pressure sensor/other detects something abnormal, and relays the alert to other systems on the same power grid. An alert pops up on a monitor in your base, you get an alert, (optional, enabled by default) and you can then either send a team out to investigate, gather more data, or start shooting with a mortar or ten.
Cameras track the movement of targets in real time, allowing you to hit mortar shots much more consistently.
Flares guarantee a mortar hit on a nearby target, but blind nearby pawns.
Etc. basically, make playing defensively inside and outside more interactive. Oh, you're perfectly safe in your mountain, but blind to the outside world. Want to look around? Gotta run some cables out there. You on the other hand are worse for wear but you can see threats coming much more easily. Stuff like that.