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#.1..0..1.#
#.........#
#.1.....1.#
#.........#
#.........#
#.........#
#.........#
#.........#
#.........#
#.........#
#.2.....2.#
#.........#
#.2.....2.#
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####D#D####
# Wall
. Floor
0 Coffin (where better place to bury a king?)
1 Item drop (airlocked)
2 Creature drop (dual airlock)
D Door
Item drop airlock design
#### F2
#X..
####
#### F0
#H##
####
X Empty space (hole)
H Hatch / 1x1 bridge (link to a lever however you like)
vertical view -
# F2
# ## F1
#_## F0
Creature drop airlock design
See above, just put a hatch/bridge over the top so the dorfs walking over the top of it don't spaz out, and make sure that however you hooked up the bottom part, it's sealed off with a wall so building destroyers can't escape.
A few notes.
1) Magma-proof crafts, coins, and furniture will allow the room to be flooded with magma, although this will prevent the introduction of (most) guardian creatures, save magma/fire men, certain irate clowns (which are rather difficult to functionally pen in anyway), and imps
2) Filling the vault with one (or more) Z-levels of water is possible, with the detriment of reducing live creatures introduced to piles of bones, and the benefit of allowing carp. It should be noted that undead guardians can still be introduced, although an additional Z-level without water must be added to prevent them from swimming back up through an airlock.
3) As per norm, the entire thing can still be suspended in lava/magma, be it in an expanded cavern built into a magma tube, a carefully crafted oceanic chamber, an exceptionally large chasm, or what have you.
4) Also remember that trapping/flooding the approach to the vault can also be an effective defense. Careful flooding can force potential thieves to hold their breath through a winding maze of water-filled tunnels, traps, and so forth.
5) Don't limit yourself to one chamber! See the below design for some inspiration!
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#.................#
#.###############.#
#.#...D.....D...#.#
#.#####.....#####.#
#.#...D.....D...#.#
#.#####.....#####.#
#.#.............#.#
#.#.............#.#
#.######D#D######.#
#......#.#.#......#
########.#.########
Primary chamber flooded with magma, decorated with steel furniture, crafts, armor, weapons, and coins; smaller chambers holds non-magma safe objects such as fine art, heavily encrusted crafts, artifacts, and coins. The entire thing is surrounded by a pool of lava, rigged with a failsafe pressure plate in each chamber that can destroy the entire thing.
Also worth noting is that the purpose of the airlock design in the item drop is to prevent looters from using this as an entrance: if the bottom is at least 2 z-levels from where items are dumped, creatures that enter cannot flip the lever to let themselves down. If the door is open, the fall should (ideally) be fatal. Really, the hatch only needs to be there at all if you plan on filling the vault with guardians, as it will prevent dwarves from spazzing out while trying to add more items to the vault.
Similarly, the creature airlock can be arranged such that each successive 'layer' of bridge/hatches prevents each fall from being fatal, in addition to preventing would-be looters from climbing up and pulling switches themselves.
So really, the only reason to have an entrance at all is if you want to actually make this vault a tomb (possibly excluding the unfortunate craftsdwarves tasked with carving it out in the first place), so you can place furniture - and even then, a few sacrificial dwarves to drop down can do that if you drop the furniture in after them. It's also worth noting that the bones can really help spruce it up a bit if you want to give it a "DEATH TO ALL WHO ENTER HERE" feel.