Dodge-me traps are everyday bread, but this one works much better than anticipated:
# - wall
^ - floor + weapon trap
. - empty space
<> - stair down/up
I - floor + upright spikes
single floor layout:
#####
##^^^##
#^...^#
#^...^#
#^...^#
##<#>##
#####
side view:
-1 #^...^# <- siege enters on this floor
-2 #^...^#
-3 #^...^# -> tunnel to fort
-4 #^...^#
-5 #^...^#
-6 ##...##
-7 #...#
-8 #...#
#...#
(you get the idea...)
#III#
All floors are connected with stairs. The goblins enter, get semi-minced by traps and dodge-fall to the bottom. As survivors try to escape upwards (The traps have one glass disc each, and the fall is not 100% lethal - we don't want to kill the poor sods just yet! Plus, they don't explode, so it's easier to get rid of the corpses) they dodge the traps and fall back again. And again. Master goblins do this over 20 times, before they are fully blended in my setup. From the last 8 sieges of over 80 gobbos, a total of 4 goblins survived so far.
TL; DR: goblin siege comes in, goblin giblets and goblinite comes out. Cheap, simple and easy to implement.
As for the song, here is "Wooden pints" by Finnish band 'Korpiklaani':
There's men, underground
Who have never seen the sun
But they really know how to party
Little men from underground
Who have never seen the sun
But the really know how to party
[Chorus:]
The rise their wooden pints and they yoik and sing
And they fight and dance 'till the morning
(...)
There's also a hilarious video:
http://www.youtube.com/watch?v=OIc4VHxU7iM