A Caverns Guide

[harlequin_corps]

That's Dwarf Fortress being FUN. Caverns are no joke, breaking into the caverns is SUPPOSED to wipe out your fortress, Forgotten beasts are SUPPOSED to wipe out your dorfs by being almost nigh unkillable. That's the whole point. Same thing with lower down. Also, a REAL siege is the same thing but above ground, 100+ goblins, trolls, beak dogs running around your fortress. Same Same.
My suggestion is that, for better preparedness for the caverns, two things.
First, invest in marksdorfs. The process of getting them to work is a bit finicky, BUT, once you do the right things, making MORE squads of working marksdorfs is just a couple clickys. Actually, that's a lie, you have to (1) click to make the squad, (2) designate the armor, (3) click to make the leader, (4-13) fill out the squad with 10 dorfs, (14-18) Set the staggered training schedule using another squads already existing schedule, (19) click off the squad window, (20-21) click on the zone and barracks zone, (22-25) set the squad to use the barracks, (26-28) click on archery range zone and set squad to use the archery range. So.. yeah.. 30 clicks or so to make a marksdorf squad.
And second, is more a cavern preparedness guide. So, General rule with the caverns. If you have ANY breaches at all, in any form at all, vertical/horizontal, you need to completely disregard everything else until you get that breach walled/floored off. Take your entire army and park it on the safe side of the breach, knowing you are going to lose probably 10+ dorfs until you get the breach repaired, (you get less killed with steel armor and good dodgers which takes YEARS in game to get together). Before you start thinking of breaching into the caverns you will want to set up shoot boxes. Dig down until under the entire cavern complex and don't dig out the Z-Layer directly above. Make stairs through every column that goes completely through the entire cavern and link all the stairs together on the Z-Layers above/below the caverns. The stairs need to have solid blocks on all 8 blocks around them before you want to dig stairs vertically through the column. If you have a column that is too thin to have a full 3x3 through it, don't put stairs into it. Then, once you have at least a working portion of the cavern bored out, establishing an Internal movement capability, then carefully dig out Z layers of the columns so you keep a full spacing of wall between "inside" and "outside". Then Smooth and Fortify all those walls out, establishing safe internal shoot boxes that can shoot at things inside the cavern. Then, once you have the capability of putting fire down on anything in a general area, establish a solid gate house in a large enough cavern column to support it. You want at least 3 sets of doors in a corridor on the outside of a drawbridge that's over a dry moat, with at least 3 sets of doors in a corridor on the inside of the drawbridge. Make sure to put the doors with a space in between to help with the door being blocked open thing. Then fortify the walls of the corridor, fortify the walls of the dry moat so you can shoot anything that falls into it. Make sure everything is set, lever linked BEFORE you break the final wall pieces of the corridor outside into the cavern. And you should ONLY use the gatehouse as a final push to send your axedorfs out to administer the final blows to what's left after your marksdorfs have gone into martial trances locking them in place until whatever they see is killed. Before you think of putting a foot down in the caverns you should have complete visual/shooting capability over that entire portion of the cavern. the dorfs will use the passageways below and above the bored out shoot boxes/columns to move from shoot box to shoot box. Remember that you don't want to just VISIT the caverns, you want to OWN the caverns..