Fire Ballistae are useful siege weapons which are only available to the Ice faction in Stronghold Legends.
They cost 150 gold and in terms of their abilities, essentially remain unchanged from previous games in the series.
However, you can no longer recruit these units from the Engineer's Guild, and you do not need to place an Engineer's Guild in your castle before you can recruit Fire Ballistae from a Siege Camp.
Fire Ballistae can attack troops, enemy siege camps, enemy siege equipment as well as wooden buildings and can destroy virtually all of them. These are especially useful at taking care of armoured units on the map who may pose a problem for other units.
Because the range of a fire ballista is so long it can potentially take out several units in a group before any of them actually get anywhere near the Fire Ballista itself.
Note: Fire Ballistae do not automatically attack enemy units within range, regardless of whether you manage to place them on aggressive mode or not. In order for these units to attack other enemy units, you must manually give the attack command. In other words, you must manually fire your Fire Ballistae yourself at enemy units in order to attack them. This makes a group of these weapons extremely useful in taking out enemy units.
The Ice Queen can destroy a Fire Ballista in a single shot. If you're planning to use a Fire Ballista to take out an enemy Ice Queen, make sure she's far enough away from your units that they cannot be attacked by her. Fire can also cause a lot of damage, and as with most forms of siege equipment, melee units can perform a lot of damage as well.
Fire Ballistae are a very unique unit because they are the only form of siege equipment - from any faction - capable of attacking aerial units within the game. While they do relatively little damage against larger units such as Dragons, it's still an option worth considering if you need to move troops to somewhere on the map and need aerial protection along the way.
They can for instance, kill a White Witch in two shots, although a small group of White Witches can easily overwhelm a Fire Ballista. If you are planning on using this approach, a Cat is recommended to protect the Fire Ballistae from attacks by the White Witches.
You will need 150 gold as well as 2 peasants from the campfire before you can recruit a Fire Ballista.