If your going for an above ground fort then id suggest this:
+ + + + + + + + + + + + +
+ # # # + # # # + # # # + z+1
+ # + # + # + # + # + # +
+ # # # + # # # + # # # +
+ + + + + + + + + + + + +
+ # # # + # # # + # # # +
+ + + + + + + + + + + + +
+ # # # + # # # + # # # +
+ # + # + # + # + # + # +
+ # # # + # # # + # # # +
z0
O O O O O O O O O O O O O
O + + + + + + + + + + + O
O + + + + + + + + + + + O
O + + + + + + + + + + + O
O + + + O + + + O + + + O
O + + + O + + + O + + + O
O + + + O + + + O + + + O
+ + + + O + + + O + + + O
+ + + + O + + + O + + + O
+ + + + O + + + O | | | O
_ _ _ _ _ _ _ _ _ I I I _
~ ~ ~ ~ ~ ~ ~ ~ ~ I I I ~
~ ~ ~ ~ ~ ~ ~ ~ ~ I I I ~
~ ~ ~ ~ ~ ~ ~ ~ ~ I I I ~
, , , , , , , , , , , , ,
+ are flooring
# are floor grates
O are fortifications
~ are water/magma filled channels
| are vertical bars
_ are walls
I is a bridge
, is ground
Put marksdwarves patrolling around the grates on z+1 and hammerdwarves behind/on/in front of the bridge. Hook the bars up to a lever and pull it so they are open then hook the bridge up to a lever so it can be opened/closed. You could put some chained animals on the south side of the first and third rows of the bridge for a thief trap.
To sum it up, in this design you have murder holes, a shoot-throughable extended entrance with heavy guarding, a portcullis, a moat and a drawbridge all in one.
It is advised to build your structure over a brook.