Now you're inside the castle, and the difficulty is starting to pick up.
There are a lot of spikes here, and to make things all the more challenging, a new obstacles is introduced - spike blocks. The spike blocks have four sides, and two sides will have spikes at any given time. You need to wait for the top to lose its spikes before you can safely traverse the obstacles.
In areas with a huge, seemingly-impassable area of spikes, naturally you'll want to use the Rush Jet. Just be sure to keep it energized. The last thing you need is to fall to your death on a bed of spikes from running out of Rush energy.
Remember, you can use the Drill Bomb to destroy walls your Mega Buster shots just bounce off of.
The Segmented Room boss lurks at the end of this stage. Three chunks of a unit will fly through the room - one high, one low, one in the middle. They'll come across at three speeds - fast, medium, and slow. For the fast and medium ones, simply slide under the unit. When it's going slow, jump on top of the lower unit, so you'll be inside the unit when it's complete. The red jewel is the weak spot, reachable with the two platforms inside. During this time shots will fire at you, so dodge them and hit the weak point with Dust Crusher. The unit will disconnect and continue flying around the room, so repeat this strategy until the villain falls.