You can play as Maria if you use the key found in the second level of the game. It's in a candle in the stretch of castle where the massive bull chases you (don't take the alternate path). The locked door for this key is in the sewers just ahead in the level. Don't pick up any other items while you carry the key. Use the key on the door to unlock Maria. You can select her in your save file.
To obtain an unlimited score and a lot of lives (20,000 points = 1 life), start game as Maria and get to the Clock Tower. Use the turtle spell to create a shield around you. Water ghosts will fly to you and die when hit the shield, but they will not cause you any harm. The longer you wait, the more points you'll get. Turn off your screen and come back later for lives and points.