Moat

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v50.14 · v0.47.05

This article is about the current version of DF.
Note that some content may still need to be updated.

A moat is a simple and universally applicable piece of defense design, and often the first defense-oriented piece of engineering that players will embark upon. A basic moat is nothing more than a dug-out trench encircling your entrance with a single three-tile wide (for caravans) bridge over it attached to a lever. In the event of an attack, the lever can be pulled and the bridge raised, forcing enemies to get creative.

Creating a moat

There are two ways to construct a moat. The first is to dig out the moat level and then channel down to it from the above. The second is to channel down to that level first and then remove all ramps. In both cases it is necessary to have either a ramp leading out of the moat or a sealed-off access tunnel, to avoid stranding your miners. If you absolutely must have an absolutely clean moat, the effect can be achieved by building a support, attaching a floor to it above the ramp in question, linking it to a lever, and then collapsing the support. The floor will fall down and obliterate the ramp. Consider the following diagram which demonstrates the construction of a moat:

Defense considerations

Climb-proofing your moat

These days, a goblin can just climb down a trench and climb right back out the other side. Here are a few strategies to prevent climbing out of the trench:

7/7 water at the bottom of the moat will drown many guests, and deter others.
Magma is always appropriate.
Cage, weapon, or repeating spike traps make it less likely that your enemies will survive crossing the trench.
Block walls are difficult to climb, and smoothed natural walls are impossible to climb.
An overhang or "lip" on the moat will prevent trapped enemies from climbing out of the moat on that side. The lip must protrude 2 tiles if made from walls, but only 1 tile if made from floors.
Make sure your moat is of sufficient width that it can't be jumped over. Invaders aren't yet smart enough to do a running jump, but they can jump across a 1-tile gap.

Trees

The greatest threat to the effectiveness of a moat is the common tree. Two trees on opposite sides of a moat are as good as a bridge if their branches come within a tile of each other; and a tree growing at the bottom of a moat makes a mockery of fortress defense.

Trees at the bottom of the moat can be prevented by digging down far enough that there is no soil at the bottom of the trench, which has the side benefits of making an invader's slip into the moat more deadly and increasing the distance necessary to climb back out afterward. Alternatively, pavement, constructions, and/or 7/7 water or magma will also prevent tree growth.

There are two primary ways of preventing poorly-timed tree growth from compromising your moat: you can either pave a 4-tile-wide collar around the outside of the moat; or you can dig a moat so wide that trees on either side will be unable to reach each other.

Lethality Improvements

As one can guess, a moat can be filled with water to drown sentients, but filling it with magma is obviously far more dwarven. Although a dry moat is perfectly functional, doing so will render it more lethal to enemies that slip in by accident, especially if something waits for them down below. This is a double-edged sword, however; you should make sure there are no dwarves standing on bridges when you pull levers, and have your military engage the enemy far from your moat.

Another option is to have the moat empty, and use a floodgate controlled by a lever to fill the moat with water or magma during a siege. Make a path through the moat for goblins to walk through, then flood it and drown them. You can also add another floodgate and lever to drain the moat when the siege is over.

A moat can also be filled with dangerous animals, such as dogs, bears, alligators or the ever-dreaded carp. This is most effective for dry or water-filled moats (since the animals will not kill the hapless dwarf than falls inside by accident, although he may just die of thirst inside it), but no one is stopping you from filling your magma moat with magma crabs.

Drawbacks

Morale

It's not that a moat is bad for morale, exactly; it's just that terrified dwarves tend not to look where they're fleeing. If a dwarf gets scared, it's just as likely to flee into the moat as it is into the fortress. Whether this is merely irritating or downright Fun will depend on how deadly the bottom of your moat is, and how easy it is to retrieve creatures stuck in it.

Amphibious/Fireimmune Enemies

It should be noted that water-filled moats can be easily crossed by natural swimmers such as amphibians and/or non-breathing creatures; this is ground for a hilarious bug^Bug:926 causing giant toads to drown the elite goblins riding them as they path through your defences during a siege. Likewise, magma-filled moats will be ineffective against fire-immune creatures who don't need to breathe, although only dragons and the various inhabitants of magma are actually immune to magma's heat, and dragons may just drown inside a magma moat.

Winter

Also note that if you fill the moat with water in a climate that freezes during winter, the moat will become ineffective while the water in it has turned into ice. A deep moat filled only partway up with water will remain reasonably effective even during a freeze.

Ranged Enemies

A moat offers little protection against ranged enemies, and sending melee dwarves out across a bridge under enemy fire is foolhardy at best. You can surround the inside lip of your moat with fortifications and engage them in ranged combat, but elite ranged enemies will still be able to hit your marksdwarves through these defenses.

Flying Enemies

A moat offers no challenge to flying enemies, and should not be relied upon as a defense against them.