The layout of the Ship
The layout of the ship follows two guiding functional necessities, zero-G environment, and low gravity. Logically each floor must be designed to accommodate these necessities plus, survival, and psychological needs of the astronauts. Naturally all life support systems will be located at the lower levels along with all hardware, tools among other crucial gear for the space journey and living on mars.
The top floors are for living areas and the flight deck.
A good starship design needs to follow simple rules. Easy to use and fix.
Problems that might arise throughout the journey need to be identified quickly and be simple to solve by anybody on board. If complex problems arise, crew anxiety and stress may lead into further problems risking the mission.
Lastly, everything on the ship must be monitored with visual information available on all floors, especially CO2 levels. User interface must be clear and simple to understand.
Credit to : Subject Zero Science