Ibanez. The published MDA for circling will normally assure you of legal obstacle clearance within a defined distance of the airport. Without knowing precisely which is the controlling obstacle you should not descend below the circling MDA (especially at night) until within the approach splay on the runway you are landing on.
Of course if you have the ground in sight along your intended flight path at all times and can ensure safe obstacle clearance (day or night) then descent is at your discretion.
But on a dark night where it may be impossible to see the terrain in front and below, you should not descend below the published MDA while on base leg. If there is a VASIS serving the landing runway just remember that it may be visible on base leg at night but it will not guarantee safe obstacle clearance until you are within the final approach splay.
You should never commence descent on base at night until you are 100% sure of safe obstacle clearance. Below the MDA obstacle clearance is your own responsibility