Now that you've bested six nasty robots, all that's left is to take down the man pulling the strings... for the first of many times. To reach that showdown, though, you still have to make your way through four more stages.
Since you've already cleared the game's initial moments, the developers decide at this point that you're ready for the worst they can throw at you. Keep all of your skills you've gained up to this point in mind, as you'll need them if you hope to survive the final gauntlet and the battle with the madman lurking at the end.
Before you get started, though, here's a note of caution: if you clear a level or two in the castle and then come to the screen that offers you the chance to continue or select a stage, make sure you continue. If you choose the option to select a stage, you can revisit the robot masters or Wily's Castle. The only way into the castle is through the front gate, though, so you lose any progress you may have made beyond that point. It's definitely something to keep in mind!
NEXT: Stage One