Locked Door 1-2?

Before entering the area with the Tower Knight, there is a locked door to your left; opposite to the stairwell. Does anyone know how to open it? Or if it's even capable of being opened?

Opening that door requires a special key that is held by a rather nasty individual deeper in the castle, in 1-3. Unfortunately, he resides in an area deeper in the castle which cannot be reached yet, even after defeating the next boss. The requirement to pressing deeper into that area will become known to you after you defeat the next boss.

You will get the key to that door later on in that level, after you kill the fat official that's hanging onto the key. You'll have to kill one of the final bosses in the other levels to finish the rest of the first chapter.