Facts:
1) Any water exposed to outside in freezing climate instantly freezes.
2) Any ice with magma underneath will turn into water.
3) Magma can be removed by atomsmashing.
4) Atomsmashing can be triggered by pressure plate.
5) Pressure plate can be triggered by enemies.
Soooooo...
Idea #1:
z0(outside):
#<<<#
#^^^#
#...#
#...#
#...#
#<<<#
z+1:
#####
#####
#DDDD
#DDDD
#DDDD
#####
# = wall, . = floor with 1/7-2/7 of water, ^ = pressure plate that lifts drawbridge beneath, < = ramps, D = drawbridge that rises to the right (or a series of 1x3 drawbridges if that doesn't work) with magma on the middle 9 tiles (when drawbridge is raised)
Goblins step onto pressure plates, magma gets smashed, water freezes, goblins get encased. Pressure plates can be elevated on the level above to avoid destruction. Reset by triggering drawbridge and filling bottom level with magma.
Idea #2:
z0(outside):
#<<<##
#...##
#^^^##
#...##
#<<<#
z+1:
######
#WWWWW
#WWWWW
#WWWWW
######
z+2:
######
#DDDD~
#DDDD~
#DDDD~
######
# = wall, . = natural ice, ^ = pressure plate on built floor that lowers drawbridge beneath, < = ramps, D = drawbridge that rises to the right, W = water, ~ = magma
Goblins step on pressure plates, drawbridge lowers, magma flows onto it, ice thaws, goblins fall down and drown. Reset by triggering the drawbridge.
Will those ideas work?