Trebuchets are available to all factions within the game, and cost 500 gold. Trebuchets are similar to Catapults in that they possess similar attributes, however Trebuchets are far more advanced than Catapults.
They can hurl rocks much further than Catapults and are also much more accurate. Three Engineers are used to man the Trebuchet instead of just two used on a Catapult.
Unlike Trebuchets in several previous games, in Stronghold Legends when Trebuchets are setup, they can be repositioned into a different location ready for firing again.
To do this, select the Trebuchet and then click on the "Toggle between packed and unpacked states" icon. You can now move the Trebuchet, or if it was previously moveable, it will now configure itself in a position to fire.
This will take a few moments to complete the necessary adjustments. Note that by default all Trebuchets are recruited in their "moving" state, so you must toggle this icon to be able to deploy them. Once stationery to fire, Trebuchets will remain immbobile until you toggle this icon again.
Trebuchets can also be setup behind defensive structures and you can fire them from there as well. You will however need to pay slightly more attention just to ensure that you do not hit any ranged units within any nearby walls or towers. Another change between older Stronghold games is that you no longer need to place a Dairy Farm in order to fire a diseased cow.
You automatically have the option to hurl dead cows over the walls of enemy castles in order to spread disease. If you move position with the Trebuchet or order it to attack a different target, it will automatically begin to fire rocks again, so you must re-select the Cow icon. Often the Trebuchet will still show that the Cow icon is selected, so you must click off the Trebuchet and select it again to be able to re-enable this functionality.
Trebuchets can destroy buildings (both Stone and Wooden), walls, towers, gatehouses and other parts of enemy castles, as well as potentially any enemy units that are targeted. Note that when diseased cows are thrown over the walls, it will create temporary noxious green clouds in the area where it lands, decreasing the health of units nearby in a similar way to the toxic green clouds produced by Dwarves, although they sometimes stay present on the map for less time. Unlike in Stronghold 2, firing diseased cows do not have any affect on the peasantry of the castle or its workers, and only affects Military units.
Trebuchets are by no means as accurate as catapults, but you can still hit a wide variety of targets with them, and it is a lot easier and safer to take down walls, towers or other fortifications without being attacked yourself, particularly if you keep ranged units near your Trebuchets. It also has a slow rate of attacking, often making sieges long and brutal. Note that the Trebuchet can also be taken out by other longer-ranged missiles such as Fire Ballistae or even an enemy Trebuchet. Trebuchets can also be attacked by aerial units, but cannot attack aerial units themselves.
You will need 400 gold as well as 1 peasant from the campfire before you can recruit a Trebuchet. Unlike in Stronghold 2, you no longer need to place an Engineer's Guild before you can recruit a Trebuchet from a Siege Camp.
Tip: An excellent tactic when using Trebuchets is to recruit several and move them to various different places around an enemy castle, then let them all decimate the castle of an enemy from where they are located. All you will need to do is make sure the Trebuchets are adaquately protected against ranged missile fire. You will likely find that you can reach most if not all of the estate through all your Trebuchets.