Caves are natural large spaces with the Earth large enough for a human, or several, to fit in. Often used as tourist attractions or for scientific research, caves often have lighting, paths, stairs and barriers already in place.
Fortifications for a cave are fairly simple. If there is only one way into the
cave, just block off the entrance with walls, allowing enough room to get in and out for a human, such as a ledge to climb over (as most zombies lack climbing abilities). Caves can also be a source of fresh water which, when boiled, can provide your group with a source for months to come. Caves are also found a far distance away from cities, so the number of zombies nearby may not be a giant horde. They also have a constant temperature that hovers around 59 (depending on the cave)
Most caves in the United Kingdom already have a working walkway system, so that if you are unsure of a second entrance, you can always follow the walkways to know. Choose the caves that are tourist attractions to be sure of stability and likelihood of medical supplies and rock stability to be absolutely sure about the area.
Caves can be damp, dark places, which can lead to diseases and infections. Cave-ins can also threaten your group, either in cutting off passages or burying you alive. Being stuck in a cave for long periods of time can also cause depression.
If there are many entry points to the cave, the chances are it will be impossible to block them all off and prevent zombies from entering. Zombies who make in it can easily swarm your group as caves can be cramped places as well, limiting your chances of escape.
Supplies will also run low and if there is a horde of zombies outside, you may starve to death. Certain minerals in the water may prevent it from being drinkable no matter if it is boiled or not. (Depending on the area itself)
Not all entrances have been found in the cave. So it's best to limit the area you plan to live in, which can lead to a dead end in a few cases.
When the power shuts off the cave lights will no longer work, and it will get very dark! Be wary of this.
Watch where you are going, as most caves contain pitfalls, underground pools, and tight spaces.