The meaning of siege is when you surround a person's castle with your soldiers and conquer it.
images from: Google Images
The meaning of ballista is basically a very big crossbow, but instead on holding the ballista it is on a wooden plank or on a tripod or a stand. The ballista also throws spears also known as a bolt during the Middle Ages. A ballista is kinda like a big spear throwing machine. The ballista works by using tension.
images from Google Images
source: https://users.ece.cmu.edu/~koopman/ballista/defn.html, http://www.lordsandladies.org/ballista.htm
This is a Medieval castle, it is made out of stone. What makes this castle unique is that it is in the middle of the water. In order to get to the castle you´ll probably have to use a boat. I think what makes it unique is that there are a lot of trees and water around the castle