Any ideea how to decrypt this?

I think we'd need the original code/riddle in order to help.

That is the whole riddle.

Well....

If it is a substitution cipher, then if SO50UT1ON=SOLUTION "50" would be "L", and "1" would be "I".

Unfortunately, 51ES only translates to "LIES" if "L" 5 and "I" is 1.

Which then leaves us with an extra "0" in SO50UT1ON.

So we need to deduce additional rules.

The solution is citadel of lies.
50 = L
1 = I
101 = CI
500 = D
50 = L
51 = LI