Planning on building a simple tower for my dorfs to use in the case of a goblin assault on the exterior. Here are a few questions I have that I couldn't find in the wiki...
1) Archer range, how large of an area can they defend.
2) Do archers need to be on the same z plane to shoot enemies.
3) How good are fortifications at stopping the enemy's attacks.
4) How can I train up my archers skill.
5) Is there any way that the enemy will be able to bust into this tower to use it to access my Fortress.
My experience with archers is almost entirely 40d, because they were so buggy in early releases of DF2010. Right now I have a squad of archers in training for DF2010, and they apparently do train sometimes, but no actual combat experience.
1) They can defend a very sizeable area. On small maps (2x2 embark squares) I could control the entire map with a handful of towers. Each had a tiny squad (3), but they were damn effective.
2) No. Mine never are. They're always on the second floor, behind fortifications. As they also always have moats putting them in a tower is probably actually a dumb idea, but shooting from towers is WAY cooler then shooting from ground level.
3) Against normal enemies they have never failed me. The trouble is elite crossbowman. Fortifications are useless against those guys. Fortunately those guys are only squad leaders, and they don't appear often. When they do appear you have to scramble, tho. Otherwise you'll lose your archers.
4) My three archery targets are apparently being used for some training in my current fortress. I made a barracks and assigned my archer-squad to it, and occasionally I see extra bolts lying around there.
A strategy I used in 40d that will still work is an arena. You make a special room enemies can't get out of, but your archers can shoot into. The easiest way to do this is make a normal room, but put fortification on one of the walls. Then you put captured gobboes in their, hook their cages up to a level, station your squad outside, and massacre the poor guys. You'll have to disarm archers, and Trolls could break out even if you lock the door, but everybody else should die easy.
5) In 40d my standard garrison tower was impenetrable. There was a moat, no door, and entrance was solely through stairs to the main fort. The moat was usually dry.
Gobboes can't fly, but they can ride flying creatures. You can minimize the ability of flyers to get into your fort by building roofs, but I have yet to find a way to build a roof without at least one square of access for builders, which means one square of access for flyers.
Again this shouldn't be a problem very often, especially if you plan for it. They don't usually bring more then one squad on Giant Bats, and a squad of 7 Crossbowdwarves and 3 Axedwarves should be able to hold out long enough for your the rest of your troops to arrive. In theory. Remember: No combat experience with bow-dwarves in DF2010.