Larger area than you think. Encompass room to grow. Claim some area to cut wood, fish, mine, etc. (You can also consider that if you've dug out the ground immediately below the surface, no trees will grow there.)
I'd make it 3 tiles thick, with a passage down the middle. That, or 2-wide, with an open half-hall, like a walkway facing inward - very Spanish mission style.
Also, have concentric circles - an outer ring wall and a main curtain wall. Redundancy for "just in case". (This can be incorporated into creating a killing field, by putting one main entry 180 degrees from the other - enemies have to run the gauntlet between the walls to get "inside". Seal the outer when it's clean-up time. More complex, w/ additional walls, to create "cells" in that middle area, for bonus points.)
(This also allows guard animals to be placed "deep" within the entry, safe from casual missile fire, and where thieves , and any ambush will be too deep and can be locked in before they can flee.)
Follow the lay of the land, avoiding allowing the enemy to have any higher positions (where they can shoot down - a 3-D line of sight exists in DF) and put fortified positions at the corners, and every 20-30 tiles or so along longer walls. If not a tower proper, than a... (urgh, can't remember the word - a jutting fortified position... someone help?)
(edit) - not a redoubt, not a redan (closer), not a bartizan (yes, but too specific)... a salient! (phew!)
Something like this:
wwwww
w w
wwwwwww w
w
www
w
w
w
.
.
w
w
www
w
w
www
w
w
This gives archers a way to shoot at the base of the wall (which rarely happens in DF, but looks "right"). Fortifications along the edges.
Stronger fortified gatehouse, perhaps even one outside beyond the main walls. Classic would be with 2 drawbridges, one outside the gatehouse, another between that and the main walls. (Sky bridges to gatehouse recommended, w/ drawbridges there optional.)
If you want to go towers, then one as your gatehouse, a pair bracing the main gate, one on each corner, etc. Maybe a very tall one in the middle. (In DF, height actually hurts range - loss about 1:1.)
There's a good simple model for an archer tower in the wiki: "Tower". Helps visualize.
Also, consider your entrance - an L shape prevents enemy archers. Look at the various "defense" articles in the wiki - some good thoughts (or seeds for them) there.