Enchanted Orbs are excellent siege devices which are only available to the Arthur faction in Stronghold Legends.
They cost 200 gold, but are well worth this amount especially if you are playing against a player who is currently the Ice or Evil faction, as these units are often a lot more powerful than the Arthurian units.
Enchanted Orbs only have a relatively short range, but their impact, especially on special units, can be utterly devastating. The Enchanted Orb will attack by releasing a large ball with spikes and letting it roll quickly towards enemy units.
The ball will continue to spiral for a while and then eventually explode, although it's not usually the explosion which does the damage to enemy units, but the rolling. Enchanted Orbs have an astonishingly massive impact on Frost Giants, depleting almost all of their health in this way.
Polar Bears are virtually killed outright while it's actually standard troops which take the least amount of damage. Lighter troops such as Men-at-Arms and Archers will almost certainly die from being knocked flying, but more heavily armoured units, or even just Macemen, take very little damage even despite being knocked flying.
Enchanted Orbs cannot attack aerial units in any way, and cannot defend themselves against such attacks. They can however destroy gatehouses or walls, although not towers. The process of using Enchanted Orbs to destroy a gatehouse or wall is slow, although you can use multiple Enchanted Orbs at the same time to be able to speed this process up.
Because of their short range, this also makes them highly vulnerable to ranged units. So if you're planning on attacking a gatehouse or section of wall, make sure that you use at least one Cat for protection as well.
You will need 200 gold for Enchanted Orbs, along with 1 peasant from the campfire before it can be recruited.