I wrote this to get some ideas for abstract combat within mechas, but it should work well enough for abstract combat within any constrained abstract environment where individual rooms aren't too huge. Assuming that it's actually functional, of course. I ripped it off in like 10 minutes.
Combat in Mechas is unlikely to use a standard grid, as the interiors are intentionally abstracted in order to simplify things. When you build a mecha, you should be slapping the cool parts together, not making a blueprint filled with all of the minutiae that will probably never be relevant. If you have a grid and a floorplan, more power to you. If you don't, here are some guidelines for abstracted tactical combat.
Compartments: Compartments are the rooms within a mecha. Assume that each compartment is a roughly a cube. Decide which compartments are connected to which, and whether or not they're connected to the outside of the mecha. Not all compartments need to be connected on the interior (some may only be accessible from outside only). Don't worry if the result makes no physical sense and would look like an Escher painting.
Hatches: Hatches are the doors between compartments. If two compartments are connected, they connect through a hatch.
Movement between compartments: Moving from one compartment to another connected compartment takes movement equal to the average of the two compartments' spaces, rounded up to the nearest 5 feet.
Combat between compartments: A character can be "at" a hatch, "near" a hatch, or "away" from a hatch. 1/8 of the characters that can fit in the compartment (rounded up) can be at either side of a hatch at a time, up to 1/2 of the characters that can actually fit in the compartment (rounded down) are away from the hatch, and everyone else is near the hatch. For example, in a Huge cargo space with Medium creatures in it, you could have up to 1 of them at the hatch, 1 near the hatch, and 2 away from the hatch. You can fit up to twice as much in each position, but such creatures are squeezing. Now, as for attacking between compartments, that depends on where the attacker and defender are on their relative sides.
Attacker | Defender | Melee | Cover |
At | At | Adjacent | None |
At | Near | Reach | None |
At | Away | No | Cover |
Near | At | Reach | None |
Near | Near | No | None |
Near | Away | No | Cover |
Away | At | No | Cover |
Away | Near | No | Cover |
Away | Away | No | Total Cover |
Melee: Adjacent means that both characters are adjacent (within 5 feet of each other). Reach means that characters are 10 feet away or further. Don't sweat the details, just go with the folks who have longer than normal reach being able to attack in melee. I might get to actual rules about how far it counts depending on the compartment sizes later.
In short, if you're away from the hatch, you have cover unless your opponent is also away, in which case it's total cover. At and at can melee with each other, at and near need reach weapons to melee, and near and near can't melee, period (DM might allow it if someone's go ungodly reach, but big reach usually would require being too big to fit in the compartment in the first place). If everyone's got big reach or is using ranged weapons, then being at or near the hatch makes no difference except to decide who is up front and thus gives soft cover to everyone else.
You can change where you're at with respect to one hatch with a 5-foot step, or with respect to all hatches as a move action.
If you're at a hatch, you can step through it, become at the hatch in the other compartment, as a 5-foot step. Anything else is a move action. If you're moving through multiple compartments at once, see the rules above about moving between compartments.
If there's more than one hatch, you can't be away from more than half of them, but otherwise anything goes.
Combat within compartments: If the combat is constrained to a single compartment, assume that everyone is in melee with each other. Flanking occurs when one side outnumbers the other by more than 2:1 (so 3 vs. 1, 5 vs. 2, 7 vs. 3, etc.). Effects that summon a creature to flank (Burning Ember, DW1 boost) don't guarantee that flanking is possible, but the extra body helps everyone on your side to flank everyone on the other side.