The gold interior
Now that you've made it into the castle, you have to work your way to the place where the evil magician Malkil is holed up. In order to do that, you must find the blue key, which is places in a very high location relative to the entrance. You begin in the lower right corner of the map. You have three paths to choose from. The lower left path leads to the exit guarded by a knight who demands 200 diamonds. You can only follow upper left path so far before you get to a gap between platforms that is too far to jump. That path is actually an exit path that you will return through. That leaves only the right side to explore.
You'll have to start jumping up from platform to platform, watching out for flying demons and bone throwing skeletons. After jumping up through a gap in the ceiling, you'll see a red treasure chest to the right (A), along with a red door behind that. You'll have to keep climbing quite a bit before you reach the end of this path, where the red key is placed. Collect it, and return to the chest, which contains diamonds, and the red door, which leads to the blue castle interior.
The blue interior
Once again, you begin at the bottom of the blue interior. There are a number of blue skulls that you can only remove by kicking with the Boots of Force if you have them. Not far off the floor, you can easily collect the pink key resting on a small block platform. Then continue climbing, making sure to visit the upper right corner where chest B is, and collect the diamonds it contains. Then cross over to the opposite side, and access the upper path on the left. You'll find chest C which contains another opportunity to collect the Feather of Feather Fall, or diamonds if you already have it.
Directly above that is a red treasure chest (D) which has the Shield of Protection if you haven't collected it by now, or more diamonds if you have. Continue climbing up the narrow passage, and you will eventually pass a blue door on the left. Keep going, and you will find the blue key which opens the door, and returns you to the top portion of the gold interior map.
Back to gold
You return to the gold interior at the top. Cross over to the left to begin dropping back down to the bottom. Keep collecting diamonds and fighting off demons. Eventually, you will reach a blue chest (E) which contains yet another Shield of Protection, or more diamonds. From the chest, head over to the right and continue dropping down. When you reach the next blue treasure chest (F) which contains diamonds. You will now be close to the original entrance. Continue to the right and drop down to the bottom level. Then head to the left. The two chests at the bottom (G and H) both contain diamonds as well. By this time, you should be well beyond the required 200 diamonds, and should be able to slip through the blue door that leads to the boss.
Boss: Growing Skeleton
When you first see this boss, you may be a bit surprised to find that it is an incredibly small skeleton. It will run back and forth, tossing bones up in the air. It can jump, but it jumps infrequently, so you are relatively safe with sticking to the usual strategy of running to one corner of the room, turning around and attacking, and then jumping over it to run to the other side of the room. The difference comes when you actually defeat the boss. Instead of dying, it actually grows and gets a new meter of health. So you repeat the process and whittle its health down when it grows a second and a third time. After it reaches it's fourth and largest size, you can truly defeat it, but at the larger size, it's a bit more challenging to jump over. You'll have to do your best to time your leaps over its head. If you have to choose between getting hit by the skeleton or getting hit by the bones, chose the bones; the bones only remove one bar of your health whereas touching the skeleton removes three. After you destroy the fourth size of the skeleton, you still have to defeat the souls that appear before picking up the treasure pile, and running through the door to rescue Candida for 50,000 bonus points.