Salting (or mudding) water and spirits (lowers soldiers morale, too!)
Burning stuff, like houses, tents, horses in stables. Killing pack
animals. Stealing anything worthwhile, like ropes, armor, weapons.
Killing their servants, dogs, plants. Poisoning stuff.
I don't know much what they did in medieval sieges, but I suppose
these tactics would be used amongst others. (Oh, and the classic.
Putting the bodies of their dead amongst their foodstuff.)
Panu Pasanen.