An unseen servant is an invisible, mindless, shapeless force that performs simple tasks at your command. It can run and fetch things, open unstuck doors, and hold chairs, as well as clean and mend. The servant can perform only one activity at a time, but it repeats the same activity over and over again if told to do so as long as you remain within range. It can open only normal doors, drawers, lids, and the like. It has an effective Strength score of 2 (so it can lift 20 pounds or drag 100 pounds). It can trigger traps and such, but it can exert only 20 pounds of force, which is not enough to activate certain pressure plates and other devices. It can’t perform any task that requires a skill check with a DC higher than 10 or that requires a check using a skill that can’t be used untrained. This servant cannot fly, climb, or even swim (though it can walk on water). Its base speed is 15 feet.
The servant cannot attack in any way; it is never allowed an attack roll. It cannot be killed, but it dissipates if it takes 6 points of damage from area attacks. (It gets no saves against attacks.) If you attempt to send it beyond the spell’s range (measured from your current position), the servant ceases to exist.
Can a Unseen Servant go through a narrow passage? For example could it go under a door and lift the doorbar out? Pick up the tiny man on the deck of a ship in a bottle?
That is a really, really good question. I'm sure the RAI answer is no but by RAW, unless the door was airtight the force should be able to walk through the gap betweenthe door and floor.
If it were allowed then the game world would likely have door bars that weighed more than 20 lbs to stop invisible servants just opening them. It also begs the question, do stairs block invisible servants? They are shapeless and cannot fly or climb. So they must be in contact with the ground and cannot navigate vertical or steep surfaces. Also how tall are they?
I think the most sensible thing would be to say they are medium sized due to str score not being modified for size and cannot squeeze through tight spaces as that would require a DC30 escape artist check.
The servant is not a creature, and so has no size. It can go through cracks and such. Biggest problem is the limitation on range. It must remain in close range, but only moves at 15'. Are you willing to go through the dungeon at that pace?
Opening a door by lifting the bar on the other side is surely a routine thing, so anyone who is preparing a fortification against a caster will use a bar heavier than 20#. I expect any gate or portcullius will have bars that are heavier. Simple private rooms, however, will not as those bars are meant to be dealt with by a single person, and likely not a muscle bound servant.
It can also pick up the tiny man in a bottle provided he is not too heavy.
As to stairs, they can cost movement by being difficult terrain, but they do not require climb checks, so an Unseen Servant can use them.
The spell is Conjuration (Creation) with a non-instant duration:
If the spell has a duration other than instantaneous, magic holds the creation together, and when the spell ends, the conjured creature or object vanishes without a trace.
Summon Monster is Conjuration (Summoning). Neither Summoning nor Creation mention anything about line of effect being needed after the cast. Since summoned monsters can go all over, so can the created force.
One limiting factor on getting it to open a door from the other side is that it is mindless and, at least in most cases, you presumably cannot see through the door, except for maybe through the keyhole or around the edges of the door frame if there are any gaps.
So if the door bar has anything securing it in place that you don't know about, that would complicate matters.