Personally I go for dry moats (at least 1 tile worth, but maybe several) against walls, as an overkill defence when either works (when not dealing with fliers).
____ W___ W Wall
####___#### _ Walkable surface
########### # GroundI often make use of the wall-top for shooting/observing/patrolling purposes but put an offset fortification to prevent falling.
H_.... H Fortification
____ W___ .... maybe floored
####___####
###########And then there's drawbridges
H_....
____----___ - Drawbridge down
####___####
###########
H_....
____ |___ | Drawbridge up, acts as wall.
####___####
###########At some point there's a de-ramping exercise. Depends on whether I want enemies that fall into the moat to get out or stay trapped as to whether I've removed the external ramps at all, internally it's academic with walls blocking the top, but it's more aesthetic to me.
I've also done far deeper moats.
H_....
____-----___
#### ####
#### ####
#### ####
#### ####
#### ####
#### ####
#### ####
#### ####
#### ####
#### _--___ Final escape tunnel
####_/#_#### Connected to a raised platform by a raising bridge, for additional security.
############ Works well with sections of wider moat under drawbridges are for falling trap.(That's the neat "no miner left to die" method. YMMV, in either neatness or in the miner-death aspect. Less neat is the exit stairs/ramp finally being capped by walls/etc, less 'nice' is to have the miner(s) deconstruct same from the top down until trapped at the bottom. Carve a slab.)
But then I tend to over-engineer these things. In advanced expectation of [CONSTRUCTION_DESTROYER] tags, or whatever.
Having said that, I don't put water in (intentionally!). If I
were to put water in the moat, I'd make it at least one less Z-level deep as the moat is. Not that this'll necessarily drown anyone that falls in (maybe install some bridges to seal the surface off on demand?). Also makes goblinite recovery, if that's your thing, less easy. Although floodgated/hatched drains and such might work, as well as help keeping the water-level what I want. But personal preferences/wants/needs/possibilities abound here, as to how you'd go about this.