Lighthouses, years ago, were built to stand up during the many storms, having to be built so close to the coast. They are isolated and built like medieval towers, which means that they are perfect fortress like structures which can hold off the undead. Also, being built close to cliffs, off the mainland, or along narrow stretches of land means that holding off the approaching zombies will be easy. Many lighthouses also have windows which can be removed to create sniping positions, and many have a spiral staircase, making it easy to hold off the undead should they break through. During the medieval wars, an attacking army was at a disadvantage fighting up spiral staircases.
Another bonus is that lighthouses are close to the sea, which means that fishing is possible. Also, many older lighthouses were built to house people, mainly one or more lighthouse keepers and a family or two. Island Lighthouses can also be found on large plots of lands, making it possible to farm crops and raise animals.
There are many disadvantages with lighthouses. First, there are not many old lighthouses nowadays, having been replaced with modern, smaller ones which only need a computer to run it. Lighthouses also do not have a large food store, and even those with some are very small. Some of the island lighthouses are lucky to have a farm with them, but not all are supplied nowadays.
Fresh water is also a concern, unless there is a well, water tank, or some means to create distilled water. Being close to the sea means constant danger of not only the undead getting washed up on the doorstep or banging at the door, but the constant danger of storms, rough seas and strong winds. This can also bring sickness from the cold and wet enviroment. Fishing can also be difficult, since many lighthouses are built by rocky shores.
The United Kingdom is one of the few exeptions to this rule, and there a few very isloated lighthouses that are only accessible via a helicopter or at low tide, these lighthouses have enough food and water to supply a single person for many months incase of problems with the helipad. Some may have a reliable supply of water depending on how important they are in the twenty-first century, meaning that they have access to an underground well less than a mile from the lighthouse itself, or a water purification system using sea water and a heating system. Even if they seem to rely on electricity, they have a back up generator that uses gasoline. Choosing to use one of these lighthouses is a more of a stop-gap measure rather than practical choice for survival.