Pitch Ditches are trenches that require pitch to be created. They appear on the screen as a tar-like ditch. Archers can light them, if they have access to a Brazier. The pitch, and everything near it, will catch fire spectacularly, burning anything near it. When unlit, pitch ditches are harmless, and units can walk right over them.
Pitch ditches require one jug of pitch to create two squares of ditch. Unlike moats, they take no time to deploy.
However, if Pitch Ditches are placed close to your buildings they can sometimes spread to them as quick as they do your enemy. To ignate the Pitch Ditch, simply select an archer nearby a Brazier and click on the Pitch Ditch.