Moats

[TinCow]

Yes, most moats were dry. Tunnelling and undermining was a popular method of dealing with all fortifications, even if they had a dry moat. Wet moats were often intentionally designed to thwart tunnelling (in addition to the other difficulties they posed) as attempts to tunnel under the moat would often result in a flooded and unusable tunnel. However, wet moats were rare because, as noted above, the water had to come from somewhere. You needed to be near a river or lake of some kind, or in the extreme example of places like Mont Saint-Michel, on a tiny island. Wet moats were indeed far more difficult to overcome than dry moats, but then again most sieges were resolved by attrition, not direct assault, so starvation remained just as effective as it was against dry moat targets.

[Mangudai]

Usually the siegers would use mantlets to protect themselves while filling a moat with dirt. If anybody is feeling ambitious, Wikipedia needs an entry for mantlets. I always pictured something like a covered battering ram without the ram.

http://en.wikipedia.org/wiki/Mantlet