Actually, that's close to what we have in some areas, down in Tucson Sector near Douglas, AZ for example. Right on the border line there are regularly-spaced towers with thermal, low-light, and visible-light scopes. They're remotely monitored, so people don't have to be present in them. One problem they've had is that some are in pretty remote areas, so they've had incidents in which people climb up there and bash the hell out of the equipment. Maybe remotely-operated machine guns for tower defense would be a good idea.
Nope, bad idea, and totally unnecessary. All you have to do is keep the towers at full height until sensors indicate movement on the support structure as somone tries to climb the tower.
Just before the 'someone' reaches the platform that the sensors are resting on, the hydraulics fail and the whole tower collapses. You know how shoddy defense contractors' work is supposed to be, right?
Same result, no liability.