It is recommended to build a small house. A good location is on top of a small hill, so you can make an underground workshop with bunkers to snipe/shoot. To make a good bunker or sniper nest, make a 3 block high wall, but cut a hole in the middle. It is a good idea to make your house pretty high, around 5 blocks, with a castle-like wall to prevent zombies from jumping up. It would be wise to make it out of sturdy materials like Bloodstone to prevent it from getting destroyed by dragons (in endurance).
A Spawn stronghold is often Located near the Spawn Tower, it's the most useful shelter in case if the player or squad is too afraid to explore further out to get resources, Iron walls and anysource of block to build it is necessary for Any survival Situation.
Sometimes you may want to build outposts with visible markers to remember spots or to simply take shelter and for future use. It is recommended to build one around 2000m, at the mountains to use as a shelter, storage, or a marker. You can also use them as sniper nests. These are usually secondary and smaller than your main shelter.
These types of structures are massive, resource intensive buildings. These would only be recommended for those that have been in a world for a large amount of time. A castle would be a large house, probably with many rooms and firing ports for defense. Castles would normally be built to show wealth to other players, and would most likely have strong supporting walls made of diamond or bloodstone.
Bunkers or Panic Shelters are often made poorly due to the lack of time of building an actual shelter. There size can be ranged by 4x5, 3x3 or 2x3 due to the fact if a Player or the squad starts to get overun by the Undead and have no choice but to make a quick shelter in order to avoid dying.