So sniper powers instead of patrols.
If you want to be "safe", yes. At least to start out (see bottom.)
You want to be "bigger" than the group you attack, right? How big will that group be?... right - you have
no fricken idea. Might be a group of 5 wrestlers, might be a group of 8 archers, might be 5 groups together - no idea. Might even be a siege party, and then you're
really up the vent without a bauxite canoe.
Worse, you, as a player, won't get
any announcement until one of your dwarfs dies. Fun fun fun.
For an airtight tower, dig a small channel outside the door and put a drawbridge over it - [BUILDING DESTROYER]]s can't get to the bridge, all good. A few traps for when it is open, better and better.
The obvious drawback is deployment, and range - a marksdwarf shoots about 20 tiles less height diff (more or less, both reduce range), so a grid of towers about every 35-45 tiles are necessary (gobbos aren't smart, they won't sneak precisely between towers.) But that's not actually that hard, not even on a 6x6 map - maybe 3 dozen towers should do it.
Besides, ya gotta do
something with all that dwarfpower.
However, it's also true that a really burly dwarf, with a decent weapon*, fully layered *steel armor* (or better) and Legendary weapons and shield skill (Legendary Armor User just helps him move faster if his Strength is low, but he'll have ~some~ by then) can take out a squad of gobbos by themselves without breaking a sweat. An entire siege if he's really worth his salt.
(* By the time he hits Legendary weapons skill, a successful hit with any weapon is either going to be an instant kill (to the torso or head) or take a limb off on 99.9% of the creatures you meet. (Some megabeasts are the exception.) Wood, steel, artifact adamantine - the actual weapon itself stops mattering when it's just overkill.)