Heliman... one good siege will get through that. Each squad is roughly 15 members, two squads will be enough to work their way through a 10x3 path of traps, even with them giving up after enough are killed.
As for military clothing, I've found that soldiers will deal with their clothing, but only if they have some place to put it. The most common mistake made by players over military clothing is to have their soldiers sleep in the barracks. Since they don't have a room, not only are they more prone to tantrum spirals when their friends die (unless you let them spar post legendary) but they have no possessions like cabinets to store worn out or unneeded clothing.
Also, military dwarves consider armor to be clothing. If you have a dwarf wearing platemail and you assign him to wear nothing suddenly he'll run off and collect enough of "his" clothing to be decent, presuming he has any. If a wrestler removes a piece of clothing but the dwarf remains "covered" by anything, be it armor or another piece of overlapping clothing, they usually won't bother to put it back on.
(Vanilla) Invaders will flee once you kill approximately half of their number.
|MMMMMMMM |
oFFFFFFFFFFFo
^^^^^^^^^+++oBBB
^^^^^^^^^+++FBBB
^^^^^^^^^+++oBBB
oFFF=====o+++o=
MMM |DDDDD|
|DDDDD|
|DDDDD|
|DDDDD|
ODDDDD|
^ - Trap. For ambushes stonefall and cage traps will work, for full-blown seiges you will want the last four rows to be maxed-out weapon traps (or more. More is aways better if you are really worried about goblins getting through)
B - Parts of a Wooden Ballista, with bolts behind. Excellent for killing tighltly-knit goblins or pesky elves. (Warning, Seige Operators are Civilians and will run from goblins)
F - Fortification.
M - Marksdarf locations and ammo stockpiles. Make sure there are walls behind them!
D - Trade Depot.
This compact entrance design is more than enough to repel any group of vanilla seigers, if you see the need. While it's nothing compared to some that I've done in the past. (If you think Goblins and regular undead are bad, imagine Skeletal WHALES. Size 16, nobreathe, nopain, noexert that can walk through four tiles of maxed-out weapon traps and keep going like it's nothing.) If you want to get a little more exotic/large, try incorporating cages of war dogs, raising bridges, melee dwarves, magma, and running water.
Also, well-equipped multi-legendary champions make Seigers cry. One or two years of training and a dwarf can punt goblins across the map with his bare hands, much more when he carries a hammer.
In my experience, however, sieges tend to follow singular paths - that is, they all path in almost a straight line, especially when their formation strings out like if they had to turn 90 degrees. In addition, when they retreat more will path through the traps on their way back. Maybe three squads, it's still very vulnerable.
Also, he mentioned only stonefall and cage traps. Weapon traps are much more useful, if imbalanced. They have about a 20% chance to stick, so a 10x3 path of weapon traps will
usually stop a siege. If you hit that 20% every time and the goblins path across single file, which will only happen once every roughly ten million times, two squads could pierce weapon traps and be into the fortress proper. Weapon traps are absurdly powerful right now, but don't rely on them. You'll regret it when one of your legendary crafters dies doing the entrance dance because you had no military to hold the enemy back. Traps are meant to be a supplement, not a major defense.
A few legendary champions will be enough to protect a fort from goblins. Draft four or six (go for even numbers so there will always be sparring partners) dwarves, assign them shields and armor, and let them wrestle til legendary. Between wrestlers and traps, anything short of ranged weapons will probably not be a threat (I would say no threat, but I lost two multi-legendary champs to a non-legendary orc before, random combat is random). If you intend to use no traps, train them in weapons. Marksdwarves are very powerful, but expensive. I'd recommend axes, swords, or spears. Hammerdwarves have a nasty tendency to chase flying enemies rather than switching targets, which slows them down against ranged enemies.