Terrain Hazards: Cellar

That's the ticket, since this town doesn't
have a well to go into, and the GM mentioned
 the cellar, this is where we need to go.

Cellars. They are somewhat civilized, except when they are a dungeon, then they are not, right? If I use a cellar in my games, it
is often a tutorial to get new players up to speed, or probably belongs to a long abandoned house in a long abandoned settlement. What kind of interesting properties of a cellar can lend themselves to making a battle interesting? Let's take a look.

Doors

First off, Dungeon Fantastic did a series on GURPS doors, so that is an interesting thing to read. What can we do with doors? We can do a lot.

A door is almost like a wall, and it can be used for cover in the same way, except it moves incredibly easy. The linked page has some metrics on typical door defenses.

A door is all the complicated mechanical bits of a trap already set up for you. I mean, the concept of a bucket of water dropping on someone when they move a door is practically a cartoon idiom. Relatively no setup time and all it takes is something that would hurt if it fell on someone... provided that the door swings the correct direction, right?

Doors are hard to see through, if you can clear two or three doors while being pursued, it is reasonable that a pursuer wouldn't know where to find you anymore. This of course goes for monsters as well as players.

A door can usually be locked from one side or another, and modern doors shouldn't take many ready maneuvers to lock. This can be an especially frustrating realization to a player who thinks a fight is almost over.

Sundries

A cellar is a fantastic place to store stuff. This means that they often have a lot of usable supplies for players or monsters (but food might not be usable, of course, in a long abandoned cellar.) It also hints to the shapes of certain rooms. Many will be large square rooms with shelves. Shelves might be sturdily bolted down, or top heavy and very easy to tip on a player or an encounter. Gigantic wine cellars might have rows of shelves where a player or an encounter can easily shoot through without being seen or countered easily, like an inside, underground portcullis. A penalty for shooting through is only fair, but because of the likely small ranges of a cellar, a fair penalty of -3 or -5 for the awkward angles is probably also insignificant next to nearly no range penalties.

Barrels are another typical fixture for storing liquid. A regular full barrel of liquid could be approximately 240 pounds and might easily injure feet or lower extremities while being difficult to counter or jump over. It could be a real danger in a narrow hallway. Even a slow roll, according to p.558 for a homogeneous item weighing 240 pounds could do a lot of damage. The barrel likely has something like 50 HP, so for every 2 yards/second it can be rolled, it could do a dice of damage to someone's shins.6 yards/second can very easily stop someone from walking.

An alternative recommended formula would calculate HP as the cubed root of mass times 2 (or 2*(mass^1/3)) This will give the more modest value of 12 HP, requiring a roll of about 9 yards/second to do 1d of damage. Using the rules for throwing on p. 355, assuming that we can consider a barrel similar to a wagon with 2 wheels (p. 353), we can divide its effective weight by 10. With a modest strength of 11 or more, 10 yd/s is not out of the realm of possibility, giving extreme damage for the previous calculation, and modest damage for the alternative calculation, for a maneuver that will likely take two seconds, provided the barrel is conveniently in the perfect position.

Regular Geometry

Though this is more subtle, it's effect on battle is not to be ignored. Someone with good navigation sense like absolute direction or 3d-spatial sense might be able to predict which rooms are closets and which doors are loop arounds for example. This is especially important if ambushes and surprises are important to your strategy style, for either players or the gm. In the aforementioned section on doors, it can also help someone determine which doors might be best to hide behind, or also which are the best to check first before a trail gets cold. For luring people into an ambush knowing where you can hide without being seen is especially important.

Trap Doors

Trap Doors are almost similar to doors but with a few interesting nuances owing to their vertical nature. A trap door in the ceiling can hide encounters that the party can't see, and although probably easier to spot, the same can be said for those in the floor. Although normal doors can be easily blocked as well, Trap Doors can easily be covered if trying to prevent a sneaky opponent from getting away. 

Loose Floor Boards

Bonus: Everyone's favorite slapstick fixture, a slap in the face from a piece of wood not properly nailed down. A trap like this is probably highly unrealistic, but it might be thought of as a player using the swing attack of a quarterstaff on themselves. It is slow enough though that any available active defense would probably make sense for protection. If it would hit a character in the face, it might have a good chance of knockdown.

Several loose floor boards, of course, can also approximate a pitfall. Any trap predicated on loose floor boards might be avoided completely by a use of light walk. I don't think a trap door would necessarily work for a pitfall the same way because of how narrow it is, but if that seems interesting to you, go ahead and do it.

Other Thoughts

I usually run games with caves and temples of doom, so I don't often think about what a cellar can offer an adventurer. Fun thing though is a lot of fixtures from one environment make a lot of sense in another environment. Regular Geometry, for example makes sense in almost any man-made structures, or for that matter, anything built by an intelligent species, if one can wrap their head around the ideas they had when someone built a structure so.