Access basement levels without opening door glitch
After unlocking Mario, go outside and do the glitch where you climb down the tree into the water. Make sure the moat has not been drained yet and you haven’t defeated the first Bowser. Swim down low and go to the metal door that leads to the castle basement. Do not swim too high or you will be above the floor of the moat and have to restart. Go around to the back side of the door and swim through it to hear the sound that usually plays when you jump into a painting. You will be in the basement and walking underwater. Walk forward and get out of the water. All of the castle's basement levels are now available. If you drain the moat, you can come and go freely without opening the door to the basement. You can even unlock the door to the upper levels of the castle before or without unlocking the door to the basement.