It's not water, it was some other fluid they discovered in a deep, deep cavern somewhere. And no, there is no Zero-Point Energy science behind it either. The Steam Ball with the compressed liquid just contained a lot of power that's all. Like the splitting of the atom. Except non-radioactive.
For anyone whose taken a course on Thermodynamics, you would be familiar with the Ideal Gas Law: PV=nRT
Now, since we can assume that Volume, mol number n, and the gas constant R remains fixed, our two other variables can now come into play: Pressure and Temperature.
Pressure1 (P1) and Temperature1 (T1) will be the variables inside of the steam ball while P2 and T2, assumed at sea level atmospheric pressure and temperature, will be the variables outside of the steam ball:
P1, P2
T1, T2
Therefore our equation becomes:
(P1)/(T1) = (P2)/(T2)
and assume P2 = 1 atm (At sea level),
P1 = T1/T2
We know that T2 = 298 Kelvin at room temperature and P1 (The pressure inside of the Steam Ball) must be EXTREMELY high so we need P1 >> 1 atm, therefore:
T1 = T2*P1 = (298K)*(A number much greater than 1 atm)
This results in a temperature MUCH greater than T2, room temperature. This can be confirmed with boiling hot water: High pressure generally equals high temperature, and vice versa.
HOWEVER, the movie depicts the "gas" that was ejected to be a kind of liquid nitrogen, very cold, especially at the end of the film when Steam Castle/Tower came crashing down and instantly freezing all the water and water vapor around it. There are two assumptions that can be made from this observation: One, this is a substance with Latent Heat properties, similar to that of dry ice; an endothermic reaction occurs (It takes heat from its surrounding environment) as it transitions from solid to liquid to gas, or directly to gas. OR Two, the exact functions of the Steam Castle/Tower must employ some very low pressure vacuum pipes and tubes to run the entire machine therefore significantly lowering the temperature; this could make sense seeing that very high delivers of the fluid/gas, translated into mechanical power, could be possible to power the entire fortress.
So, is this technology physically possible? Maybe. Is it feasible for engineers to build? Not really. Hope that answers your question.