Dwight, would you approach a target from the obvious routes? Check your terrain and see if you can find the - not so obvious routes too. If you lack the manpower to use sentries then think about "unusual" alert or sentry systems..... think VC.
There are only two obvious routes into my lair, . . . both will be blocked, . . . people will have to detour to enter, . . . making them slow down, . . . listening devices will be near the turns, . . . they will be challenged through external speakers, . . . but I'll be spotting on them from a safe distance, . . . through a milspec scope.
If it is at night, . . . I have IR capability, . . . I should see them (or whoever is on sentry duty) long before they get close in.
Yes, . . . there will be gravels in the coffee cans on strings, . . . might even be a flash-bang or two at appropriate intersections.
The neat thing about flash bangs, . . . if they are up to no good, . . . chances are they will attempt to return fire, . . . thinking they were attacked. It might weed out those we really do not want around, . . . because if they return fire on my flash bangs, . . . well, . . . it might not be pretty when the smoke clears.
May God bless,
Dwight