From the Save Room, head right and descend further into the Alchemy Laboratory. This area introduces more challenging enemies like Flea Men and Axe Armors. Be cautious and use your shield. Your primary goal here is to find the Jewel of Open. This key item allows you to open blue-glowing doors, which are common throughout the castle.
Explore thoroughly, breaking all candles for hearts and potential item drops. You'll eventually reach a room with a large, green-glowing machine. The Jewel of Open is located in a hidden passage above this machine. To reach it, you'll need to perform a high jump or a double jump (once acquired). Once you have the Jewel of Open, backtrack and use it on the blue door you passed earlier to proceed.