Easiest way for areal denial (or mutilation) is trapping your areas with normal traps. However, if you want traps on area that can kill whole squad when first goblin steps on it, the minefield is an option.

For minefield you will need:
-Replenishing Magma
-flat area to be mined (not mandatory, but easier to implement)
-magma proof materials for mechanisms etc.

Let's designate our outside surface area (where the goblins etc. walk) as level z 0.

On z -1, dig a tunnel network under the area to be mined. I build my own with tunnels every 5 spaces. Full hollow is acceptable too, it just takes longer to get operational, but allows better mine placement.

Next, build a magma pump, which pumps into the z -1 level tunnels. Magma pump must reside at z 0 or higher level, and entryway to the tunnels must be covered on all levels below pump. Higher level pumps cause bigger "explosions". Open entries cause overflow, and make the minefield stop working.

Note for later: It is a good idea to place a kill switch to the mechanism powering pump(s). (safety)

Next step is filling the tunnels. Pump them full of magma.
This also allows you to check that no overflows are happening, when tunnels are full.

At this point, shut down the magma pumps (otherwise next step will kill plenty of dwarfs.)

Next, place the "mines". On level z 0, channel single square holes into the magma tunnels where ever you want the mine to be. (you can have multiple mines, try one and see how large area it covers. Then decide after that). Cover the channel with a magmaproof-hatch. Place a pressure plate (or multiple plates)next to the hatch , and trigger them to open hatch, when stepped on.

Re-connect the pump, and there you have it.

The operation principle is simple. Magma is pressurized from higher level into enclosed space with no exits. Hatches are opened when someone steps on pressure plate near them, and the pressurized magma shoots up, spreading into the open space around it.

If the area is big enough, then when the hatch closes, the released magma will spread out into 1 layer and disappear (though this will take a long time). Once triggered, the area is thus denied for a while. Trap also conveniently burns all those narrow clothes, but leaves iron items.

Other details.
-Allmechanisms must be magma proof.
-Changes to pumps (height, amount) can increase the size of magma explosion.
-Safety on pumps allows disconnecting the minefield so it won't kill animals and hunter following them. This allows your minefield to ignore a mairy marmot, and be ready for the siege, instead of burning the animal, and making the enemy avoid the large area of 1 deep magma.


-Allowing pressure plates to be triggered also by magma will cause a big chain reaction, and the field will not clear, unless it can be drained somehow. Remember that magma will rise to the level of the pump, so be careful or "fun" will happen.
-Alternatively, the trap can be used to really trap enemy, by placing trigger in center of area, and hatches on edges. This requires more mechanisms, but it is possible to open all hatches when plate is triggered, thus trapping the enemy unit by surrounding it with magma. (if they step on the plate again, they may release enough magma to get themselves killed.)
-Obvious place to locate one is a bit further away from your entrance, on area from where enemies approach during sieges.


Now, I build the minefield on one fortess, but stopped playing before anyone stepped on it, so the efficiency is unknown.

So. Any comments? Questions?