A floorplan is about planning the map's flow and object placement at the most basic level. On top of it we can and should include gameplay elements. The most accessible floorplan type is the realistic one. Think about streets, villages, homes, supermarkets. Those are the most known floorplans by people. It's easier to think on where to place objects, rooms, offices, etc by thinking on everyday life. The other type of floorplan is abstract, something that doesn't resemble buildings, streets or villages. With an abstract floorplan we have more freedom to think on unnatural worlds, bizarre or fantasy themes. It doesn't necessarily means easier though. To choose realistic or abstract very much depends on what type of game you are making and the storyline.

I don't think we need to use any specific software for this phase. Paper ad pencil will do it. It doesn't need to be perfect. Proper scale is more important than having exact dimensions.

With a floorplan we have a better overview of each piece of the level and how they fit together. We know how much space there is going to be, how much space a door needs to open without hitting something, how thick the walls are going to be, the dimensions of each room, the dimensions of every object that we place in the level. We can also see the path that we intend the player to take. If there are blocked doors and buttons to open them or keys to grab, we can predict possible routes and the order of each action that the player is going to take.

This abstract floorplan example is from the original doom, episode 3 map 2. Its shape is a left hand. It's not possible to notice this shape from the player's perspective, unless he or she could fly and see the map from a distance. There isn't a single reason to choose this path, it depends on many different factors. From a modern standpoint choosing an abstract floorplan can't be done on a whim. The decision has to be grounded on some research, gameplay, plot or something. In the real world we have examples of buildings or homes that were made to resemble a flower, a dog, some cinema setting or any other unconventional idea. In respect to games we have to have a good reason to do so, otherwise there is a huge risk of going for a floorplan that doesn't make sense for the gameplay, game's theme or storyline.

Imagine a house that was built to resemble a flower. If that decision was made on a whim, whoever lives in it is going to face some problems. It can be unwanted shadows, windows placed in an orientation that prevents the sun from shinning in, ceiling being too low, etc. I shall give a real world example: in London there is a building called "20 Fenchurch Street". Due to its unusual shape and its facade being covered with glass, there are periods of time during the year that the building behaves as a giant concave mirror, thereby burning anything that is hit by solar ray beams focused by the building's facade. In a game we can easily rotate a building, change entrances and materials, but in the real world we don't have such luxury. Even within the world of 3D modelling, making last minute changes are always more costly and harder to make than during the planning stages.