The Basics of Perimeter Defense
If you cannot defend it, then you do not own it. In other words, someone, or some group can come along and take it. Take over your shelter, and enjoy all of the luxuries you have stockpiled while you sit shivering in the cold rain plotting your strategy for getting back inside.
Once the SHTF, readymade shelters like the one you have will be on the minds of those without one, and there will be plenty of people without one. The ones least prepared will be forced by their circumstances to try and take what others have worked hard to gain.
You Are Always In a Defensive Position
You have something that someone else wants, and you want to keep it out of the hands of others. You are defending it, so you are usually in a static position presumably ready to repel all boarders if you will. However, to ensure you are at the ready when there are intruders, or potential intruders lurking about, you need a warning system in place, and you need personnel in a position to add fire support from outside the structure if there is an attack.
There is a difference between a spider hole and a foxhole. A spider hole is traditionally a camouflaged observation post dug in the ground. A spider hole is not considered a fighting position. However, it may become an ambush position in some cases or a fighting position if it is spotted due to poor camouflage or if someone spots a muzzle flash, for example.
The hole is just big enough for one person to squat or sit with a pack on and it has to be roomy enough for maneuvering a weapon as well. The top is concealed by what is found in the environment so it blends into the environment. A person is concealed and yet can observe trails, roadways or paths leading up to your shelter or to a structure you are defending.
The person in the hole would act as an early warning system and would provide firepower in the event the shelter came under attack. Spider holes allow fighters to conceal themselves so they can be used to flank an opposing force moving toward the shelter, or as stated earlier, the holes can be set up as ambush points, or in some cases, can be used as a temporary sniper post. Once you fire from the position however, it is compromised, and you should then seek cover and/or concealment elsewhere.
Dig the spider holes along the perimeter so when occupied personnel can observe 360 degrees around the shelter. The holes have to far enough out from the structure so that when you receive radio signals that an intruder has breached the perimeter those near or in the shelter have time to ready for a possible attack.
Choose the locations carefully because you do not want them flooded during heavy rains, and you do not want to expend time and energy digging through heavy root systems. The holes must be camouflaged, and to test the camouflage have those in the group that are not aware of where the holes were dug attempt to spot them from obvious and not so obvious avenues of approach.
A foxhole is a defensive position, a position to retreat to or a position to take up when the shooting starts. A foxhole is bigger than a traditional spider hole, and is typically dug big enough for two shooters or even more to stand up in and to shoot over the berm. It allows a shooter to stand and shoot while still offering some cover from incoming rounds and protection from shrapnel.
Foxholes located on your property would be firing positions away from the structure, but close enough to get to during a fight. If you fear your shelter might be burned or bombed then you would seek cover in a foxhole.
Military foxholes will have a grenade sump. A grenade sump is a hole or depression slanted into the sides of the hole so grenades and other explosive devices can be kicked in, to help reduce/reflect the blast. Foxholes should also have drainage sumps to keep it from filling with water. A foxhole on your property would be considered a permanent post.
You can build up one side or all sides to create a berm using the soil from the hole, or you can use sandbags to deflect/stop rounds and flying shrapnel.
You can cover and camouflage the holes when not in use to prevent them from being detected by surveillance of the property.
Defending a position is about being ready to defend. Lacking a good defense, your only option would be to escape. You have to make decisions quickly. Size up the opposing force, and decide if you can sustain the fight long enough to defeat said force.
If you had unlimited ammunition, food, water, and personnel that can shoot, then you can of course, sustain the fight. However, this will never be the case, but in most instances those trying to take your shelter or supplies will be unprepared citizens, looters, or opportunist looking to grab something quickly without a fight.