Burning Carts are siege weapons which are only available to the Evil faction in Stronghold Legends. They cost 70 gold, which is cheaper than in Stronghold 2, although their features haven't really changed.
Burning Carts are a great way to take out enemy buildings and to devastate the enemy economy with little to no damage to your own troops.
You can also do a lot of damage for example to armoured units, who move slowly and fire will cause them serious damage because of their armour.
Aerial units will also take damage from the fire they leave behind. Note however that unlike in Stronghold 2 you cannot directly attack a unit with a burning cart.
However, you can order your Burning Cart to attack the area surrounding the unit via the "attack specified area" icon, which will attack the unit at the same time.
Once you left click to attack a building or specified area, the hay is lit and the burning cart is moved towards the intended destination until it reaches it. Once there, you will have to quickly select another target to attack as well, otherwise the fire will also destroy your own burning cart.
Burning Carts can get destroyed by enemy ranged units incredibly easily, and by enemy melee units as well, so it's important to protect them until the last second when you have already attacked a target, or are about to attack a target. Men-at-Arms can throw their Javelin which will destroy the Burning Cart immediately. Should your Burning Cart be attacked and destroyed once the hay is alight, then it will leave behind fire in the place it was destroyed, regardless if it does not even reach its target.
Once the fire is left behind it will spread over a period of time and destroy or damage any buildings or units in that specified area. You will need 70 gold for a Burning Cart, along with 1 peasant from the campfire before it can be recruited.
Tip: Burning Carts can be incredibly successful in large groups. To maximise damage to an enemy castle, you could recruit several Burning Carts and send them all into the enemy estate at varying points at once. This will likely destroy most buildings in the enemy castle. Burning Carts can also be quite useful in clearing man traps as well.