Door Week continues.
Okay, there aren't really 80 ways around a door in GURPS unless you start double-counting, but I like the song by BAD.
But for players not fully conversant in the first half of Dungeon Fantasy 2: Dungeons*, here are some ways to get through doors in my game. This is basically summarizing the rules from DF2, with clarifications and notes to help you know your options for making it better.
Important Note: All of the force options are noisy. Lifting, picking, and possibly magic will be potentially stealthy, if you yourselves are also stealthy. Using any forcing method is potentially going to attract wandering monsters (more rolls or improved chances of monsters appearing), destroy the possibility of surprise, or otherwise make your presence and actions obvious.
Bashing: We all know doors are the greatest enemy of adventurers, so why not slay them? Forced Entry @ +1 or +2 gives a damage bonus, and Breaking Blow is very useful.
Wooden doors have Ablative DR, so if you keep hitting the same spot you can wear it down to a mere 1 DR per strike (stone is 3 DR, for the occasional stone doors.) Iron-bound and iron doors have normal DR, so if you can't ding it up on a few strikes you're probably not going to take it down. Only the lighter ones give you any hope of inflicting damage - an average-quality metal door has DR 25!
Bashing opens a roughly 10 square foot hole (per Low Tech p. 122); you need to bash up a double-sized spot if you're going to go through without crouching and one half again as big in either case if you want a SM+1 person to fit through without squeezing.
Assume hinges aren't modern pin hinges designed for easy removal - taking them off might be one way to get a door open but it's not quick and easy. IQ-based Forced Entry can help here.
Helpful Tools: Crowbars (DF1, p. 24) do extra damage to doors. Otherwise, use an axe (for the cutting multiplier on penetrating damage). Don't use a sword - there is a 3-in-6 chance of it bending (DF2 p. 8); fine and very fine swords hold up a bit better.
A portable ram (DF1, p. 26) is ideal. A Siege Stone (p. 30) might help, too, in certain cases. A mallet (p. 24) or crowbar is what you need for removing hinge pegs, and a work hatchet (p. 24) is a good idea if you don't want to use your weapons to open doors.
Image of a much bigger ram.
A bit-and-brace might be useful (p. 25) to make a hole to look through or to get at concealed locks and hinges. Finally, acid (p. 28) is another way to deal with locks and hinges.
And if you want to know what's behind the door, a spy's horn (p. 26) might help you hear it. It's much harder without one of those to suss out noises.
Forcing: If the door is merely stuck, Forced Entry will do the job, usually without a penalty.
For locked, barred, or wedged doors, brute force might do - see Muscling Through.
I assume you've giving it the boot or shouldering it open, and this affects your defenses as if you're doing a Committed Attack (Strong), if it matters. There really isn't an option for a "light shove" here. I'd allow an All-Out Attack (Strong) for a +1 to your ST for forcing purposes.
Extra Effort (p. B357) or anything else that boosts ST helps - Power Blow, Might spells, etc.
Helpful Tools: Crowbar (DF1, p. 24) for a +2 to rolls, but it needs two hands.
Picking: If the door isn't stuck, just locked, you can pick the lock. Generally this is DX-based Lockpicking, although complex locks and puzzle locks would be IQ-based Lockpicking. Some puzzle locks are player-facing challenges, though, and you'll have to figure out the trick to open the door.
This takes time (1 minute per attempt, longer if you want a bonus, per p. B436). The Lockmaster spell is faster, but won't work on locks proofed vs. magic (meteoric or mana-drained components.) Such locks aren't uncommon - by default all of the high-end locks are done this way.
What about grates and portcullises?
Bending uses the rules for forcing. No tools will help, although it's possible to engineer ways to get multiple people pulling on one bar (cable, space, and the old heave-ho, for example.)
Lifting a movable grate is just a question of Basic Lift, unless it's secured - go back to Forcing or Picking the hinge or lock, or Bending the bars.
We'll Shape the Walls! Of course, magic might help you simply bypass the door. Costs are for worked stone (6x, so minimum cost 12), even if the door is set in generally non-worked stone (it's worked by nature of having hinges, a lock, etc. put in it). Most walls in Felltower are worked and/or treated with mana resistant enamels, paints, and carving methods, giving a further -1 to -10 to shape. Assume designers putting in a serious door put in serious countermeasures to common and easy magic!
There are other magical ways to get around doors - Apportation might remove a simple bar, although you cast at at least -1 for range and -5 for unseen. Most non-improvised bars will have a locking handle, latch, length of cord, etc. to keep the bar from moving freely until it's unlatched. That will effectively stop Apportation unless you first cast the spell on the latch and then work it free, blindly. Undo is possible to completely remove a door, depending on the construction. And many spells can cause damage to doors, dovetailing with Bashing, above. Spells cast beyond the door are at -5 for an unseen target and normal range penalties (minimum -1.)
Mining. You can also try digging through walls and floors - use the rules under Digging, p. 350-351. Requires a pickaxe (p. 24) to break up the rock, and then a to move the debris efficiently (hands are 1/4th as fast.) Slow against stone - the door will almost always be easier, and if it's not, it's probably because the walls or floors are thick. And tiring - one hour of breaking and/or shoveling rock is 4 FP.
That's not 80 ways around the door, but it's a start. Any I missed, please ask me, point them out, or write them up in the comments. Page refs are appreciated if I missed a rule!
(Important clarification: if I missed tools from the Dungeon Fantasy books, rules from Basic Set, or completely new methods for dealing with doors, please let me know. Minor variations of the above aren't really necessary. Neither are ones that don't deal with doors, like "learn the Secret Teleportation Spell from DF11!" Obviously Ethereal Body or going around a different way that doesn't have doors would work, but it's not useful to discuss here. This about the rules for dealing with doors as physical obstacles.)
* I actually have the first half of DF2 printed out put in the three-ring binder of DF rules for my players, aka "The DF Player's Book." The first half of that book is excellent player-facing material, which really helps lay out the effect of their skills, stats, and basic choices in clear rules.