Sparda was Mundus' most trusted lieutenant and blood brother, one can assume that he was quite powerful and fearsome.
Dante only beat Mundus due to sheer dumb luck and partly because he cheated.
Sparda was caught because he did not take a stand and fight, he was more preoccupied with getting his sons to safety.
And his betrayal had been discovered, so he had lost the element of surprise when Mundus came.
If you assume that the giant chained statue is Sparda then he might be just some strong demon.
Mundus would not have succeeded in taking over the world without Sparda, that says something of his power.