The lighthouse is a standard red-and-white striped tower, making it very, very easy to spot. The only two known ways of getting up to it are by scaling the side (the spot where the body is is the easiest) or by simply taking the path (and bridges) leading to it.
While each individual floor of the lighthouse is somewhat small, there are four of them.
Ground floor: a small room containing some drawers, a cupboard, a furnace and the door.
First floor: a room containing a bed and a cabinet.
Second Floor: some form of storage room containing pallets, a good source of reclaimed wood.
Third Floor/roof: the top dome of the lighthouse containing the large revolving bulb they are known for. Gives a decently good view of the road, the lake, the mine entrance and the whale processing plant.
Note: This is likely the only self-contained Bunker in desolation point, with its storage, source of reclaimed wood (good if you're snowed in) and heat source. The one thing it's missing is a workbench.