The biggest obstacle when it comes to defending a piece of ground is resources, in particular trained personnel, weapons and ammunition. You can have an arsenal at hand, but you as one person can only fire one weapon at a time, so if you have limited personnel then the number of weapons is not a major factor, unless you expect to gather more people in the future. Ammunition for what weapons you do have however is a major factor.
People may have enough ammunition to fight their way into trouble but may not have enough to fight their way out of the problem. A sustained firefight requires a tremendous amount of ammunition, and you simply cannot deplete all of your rounds in one fight. Therefore, you have to avoid engaging others whenever possible.
Planning
First, be prepared to move quickly from your shelter to another location, and this means you need another location establish prior to any action. Spider holes, foxholes, and/or camouflaged blinds are just some examples. Your alternative locations must be such that you cannot be trapped inside them. Remember cover is protection from rounds and concealment is protection from probing eyes.
You do not want to become trapped inside your shelter. You can be burned out, or overwhelmed by superior firepower, doors can be battered down, grenades and/or other explosives can be dropped through holes made in the roof, and the list goes on. If it is just you and one or two others firing you cannot cover all sides, and the roof, so becoming trapped is a real possibility.
Ideal Circumstances
Position your shelter in the center of a circle drawn on a sheet of paper that resembles a compass face, with north, south, east and west annotated. Your front door faces south for example, so the back door faces north. Measure out a 1,000 yards from all four corners and call this your perimeter. Once you know the 1,000-yard mark break it down further into 100-yard zones to create fire sectors.
The range figures are examples only. Only you can determine the size of your perimeter based on the size of your property, and geographical features.
Ideally, you would have listening posts for at night and observation posts for daytime hours established at the 1,000-yard mark at all compass points to alert others back at the shelter of any intruders. Once the alert goes out that, intruders are on the property then those tasked with defending the shelter would go to their shooting positions. Everyone would need to know his or her 100-yard sectors. Fill out range cards for each position and leave the cards at those positions.
The range cards should tell anyone at that particular position how far it is from the front door, front or back windows and so on to the large oak tree or any landmark for example, and show the field of fire so shooters are not haphazardly firing in all directions. The firing positions would overlap each other to ensure there are no dead spots that can be exploited by the enemy. Ensure all weapons are zeroed for the appropriate ranges. Snipers can be used and of course, their weapons would need to be zeroed out to a 1,000 yards.
As stated, the above is an ideal situation where you have enough personnel to act as guards, you have trained shooters, and all firing positions are covered. This is not likely to be the case in most circumstances however, so plan “B”.
The Most Likely Circumstances
If it is just you and a spouse/partner and children and you are facing marauders, looters or others then it is time to employ guerrilla tactics. You would have to get out of the shelter to prevent becoming trapped. Once outside the shelter you will need positions to fire from, conceal yourself and/or have positions that provide cover for non-combatants in the group.
As difficult as it is, you may have to abandon your shelter to save your life and the life of your family members. This means you would need supplies cached in the area so you can survive until either the looters leave with all of your supplies or you can regain the shelter by force. Your supplies will be either stolen or damaged, so having supply caches is critical. You would have to consider the possibility of never using the shelter or land again so you would need enough supplies cached to get yourself established in another location.
To avoid engaging in a sustained firefight you need an early warning system if you have limited trained personnel. If you have electricity then exterior lighting can be used, motion activated lighting in particular, along with motion-activated sensors that can be placed around the perimeter along with cameras either wireless or hardwired. Guard dogs can be used as well.
No system is infallible so you also have to be prepared to get all non-combatants to a safe place such as a root cellar, basement or some other place that offers cover from flying rounds if you cannot escape the shelter before the shooting starts.
Things to Consider
Anyone or any group looking to steal your supplies is not likely to burn you out immediately, because they want your supplies undamaged. However, criminals are not known for their ability to reason through situations, so they may end up attempting to burn you out after a few hours of exchanging shots.
If society has collapsed entirely then self-appointed groups or groups appointed by the government may decide it is time to redistribute supplies. This group will want your supplies intact as well, so they may settle in for a long siege. They can cut off your electricity or even water supply and use other harassing techniques to get you out of your shelter and to force you to give up your stockpile.
Defending your bug-out location or safe haven and possessions during a crisis is new territory for most people, and if you do not have specific training in defense or offense tactics then it is likely you will lose your supplies or even your life if you try to barricade yourself inside and fight it out.
It is important that you have a plan for escaping any shelter when up against armed individuals or groups, so you can live to prep another day.