High towers may make you feel better, but know that excessive height hurts crossbow range. Not by a lot, but some.
Also, there is a Line of Sight rule that will cause targets to be safe if they make the foot of that tower - about 1:1 for distance:height - add something (a moat, a berm, whatever) to keep them away a bit. Or, add a lower firing balcony as well - then you can pick and choose.
Adding walls between fortifications might (?) aid in defense against massive enemy archers, but it also limits the number of archers who can fire from the tower, and their field of fire. I'd go with 100% fortifications, every time.
And don't forget a roof, over at least part of the archer's positions, if you're going to use the "All dwarves stay inside" order.
(There's a blueprint for a plain vanilla archer tower in the wiki, under "Defense design", and discussed step-by-step under "tower". It can be mod'd to suit.)
As for a "realistic" gate ("realism" being a term I always cringe at in a game based on dwarfs, ahem), I'd suggest a sharp L-bend (or two) immediately after the portcullis, to prevent archer fire from entering your compound. A short wooden drawbridge over a moat (wet or dry) would seem realistic as well, but that's your call, certainly.