40d:Moat

The term "moat" refers to a defensive ditch that optionally may be filled with water or magma. There is no in-game structure called a "moat", but any channel defensively placed to block invaders, whether around your entire fortress or just a short one in front of a bridge, can be considered a moat.

A moat is an effective way to keep sieges away from your walls and fortifications. This gives your marksdwarves protection from enemy archers, and protects your doors and bridges from building destroyer creatures such as trolls. A well-designed moat prevents the passage of any creature traveling on foot, though flying creatures, of course, just don't care.

(Remember that any creature can path diagonally across tiles, so there can be no diagonal path for your moat to be effective.)

It is not necessary to fill a moat with water for it to be effective - dry moats work just fine. While creatures knocked into an empty moat will be unharmed, those knocked into one with water will drown, but recovering items dropped into a water-filled moat is a challenge.

A water-filled moat can freeze in cold weather, allowing foot traffic across the ice. If this happens, the ice can be mined out, returning the moat to a dry state. A more elaborate method would be pumping the water out before winter and back in once the Spring thaw comes. Yet another method is to dig a 2 z-level deep channel and fill only the bottom z-level with water, which also works against swimming creatures.

A magma-filled moat is the deadliest: It does not solidify in the winter and it instantly kills all that fall in, friend or foe, along with any items. (Dwarves in combat have a habit of dodging off the edge of bridges and walk-ways unless there is a solid barrier like a wall or fortification to stop them - something to consider.)

If you wish to combine safety and offensive value, consider a channel lined with your favorite traps.

A very deadly combination is to first create a moat, then put a retractable bridge across it. On the outside of the moat, next to the bridge, put pressure plates surrounding the bridge. Link them all to the bridge, and watch as invaders step on the pressure plates, then step on the bridge, the bridge retracts, they fall in, and drown/burn.

Reverse Moat

Digging channels for a moat interferes with the top layer of your fortress. But it's possible to get the benefits of a moat without channeling from the surface. Construct two walls around your entrance, and connect them by a drawbridge. Build ramps or stairs on the inside of the inner wall, and the outside of the outer wall. To cross the moat, a creature has to climb up, cross the bridge, and climb back down. If the bridge is wide enough, and lined up with three ramps on either side, merchant wagons can enter.

Trap Moat

Essentially, the Trap Moat is an empty moat with a retracting bridge. However, the inside of the moat is filled with upright spike traps. The idea is, when enemies get on the bridge, you pull the lever to retract it and pull the lever to spring the traps. When they fall, they get perforated. However, there's ramps at the end of the moat so friendly dwarves can get out and surviving enemies can futilely try to cross the bridge before they bleed out.

Dungeon Moat

Like the trap moat, only the channel is filled with cage traps instead so you can expand your zoo. If you use this strategy, it is recommended that you connect all your moats together, so that if all the cages in one moat are full, hostiles will (hopefully) wander over to the bottom of another moat and get caught.

You will need an entrance into the network to retrieve captured enemies. This entrance should also be controlled via drawbridge/moat access, and the moat it uses can connect via stairway to the dungeon you've already built.

You might also want to include some upright spike traps in case all the cages are filled. Otherwise you could potentially have the entire enemy army wandering around in your fort's dungeon. Of course, that might be a problem you want to have.

This idea would be more fun if you had a reliable way of getting enemies inside the dungeon. For example, you could restrict entrance to your fortress along a narrow passageway, and then replace the passageway with a retracting bridge over your dungeon. (Hint: You'd have to build the retracting bridge first, then turn the bridge into the only means of getting into the fortress using channels.)

DISCLAIMER: I haven't actually tried any of this out yet; something will probably go wrong (for example, enemies that fall into the moat might be too stunned to wander around and find an open trap.)