A basic watch tower to see into the horizon, most effecient when doors are placed on land level, also allows for underground access. If the underground access isnt wanted just cut the bottom half below the doors off.
Contains 4 types of room, the blocked room with just windows, the balcony room with a balcony, the main room with double doors leading outside and the top floor with open views of all sides.

It's left as simple as possible because it is meant to be designed to the individuals taste.