As far as placing walls goes, they can only be placed on the edges of each hexagon that has a floor (all modules come with a floor in the space they occupy). Walls also cannot be placed on tiles that are inside of modules that are more than one hexagon large, only the outside. Doors can be placed anywhere walls occupy, costing nothing and adapting the type (light, sturdy, heavy) of the wall. Lower tier walls and doors can be placed if the cost for a higher tier has been paid for that hex (if you really want to).

Wall Costs

The cost of walls is applied per hexagon. That is to say, you can construct a wall on any number of the six sides of a hexagon while only paying the cost once.

Wall Health

Each segment of a wall or door (one of six edges placed on a hexagon) has its own health pool. Doors of the same type always will have less health than wall.

Airlocks

Airlocks are modules that occupy one hex and tend to have a high amount of health. They represent a room with doors on all sides that open and close so that they do not let air escape from the ship. Additionally, they are large enough to let mechs through them, and can be placed adjacent to each other to let larger mechs through. They can be occupied and passed through. When occupied the unit inside cannot target or be targeted by anything outside the particular airlock. They are airtight no matter if their space is occupied or not (unless overridden somehow).

Airtight spaces

Any space (multi-hex or not) enclosed with walls, doors, or airlocks is considered airtight and will be filled with breathable air by any onboard life support systems. Any spaces that are not enclosed are considered in the vacuum of space or in the atmosphere of the planet you are on. Spaces can lose airtight status when walls, doors, and airlocks are opened or destroyed. Anything not bolted down or fastened within a newly vacuumed space will go flying out the hole in the hull! This includes people, mechs, and any random objects in the room, all of which are deposited nearby in space. People can make an athletics check to hold onto something and stay still.