Underground Waterway
To simplify the explanation just a little, I have incorporated a map system.

This is a map of the next section. The path to the boss is marked in yellow. Follow that path if you want to just beat the boss. To get all of the extra items along the way, take the lavender path. The gray areas are hidden rooms. You must strike the wall with your whip to access them. The red dots are Heart Max upgrades. The orange items are Health Max upgrades. The light blue items are Magic Max upgrades. You can only get the upgrades that have lavender lines going to them. The other ones you will need more Magic Items to get to. The red squares are save points. Always save your game at a save point. Not only can you save your game but you can also replenish 100% of your health and magic. Be aware that I will not tell you to level up. It will be your responsibility to do so. I will also not remind you to go back and grab any extra items that you may have missed. You can stray from this guide at any time to get extra items you missed.

This one can be confusing if you try to think too hard. There are switches strategically placed throughout this area. When you hit one of the switches, all of the bridges/stairs made up of blue blocks will shift. Either going to bridge position or to stair position. For some of the extra items you must move forward a screen, hit a switch, then move back to the blocks you wanted to move. Trial and error are key here. If you are having trouble, then please email me and I will update this section to help you out. Let's start. Follow the yellow line to the second room. Grab the extra item and continue on. Manipulate the switches here and try to grab as many 3extra item as you can. The first hidden room here, the one with the red dot, you actually have the walk through the wall to find it. After grabbing all the extra items head to the next room. The hidden room in here cannot be found until you get the Roc Wing at the end of this area. Grab the other extra item and leave. Grab the extra items in this room and save your game. Head on to the next room. In this room you can take the top door or the bottom one. Take the top door if you want the extra item, then take the bottom door. Head on through and save your game. Go and fight Camilla, and show her who wear the pants in this castle. Grab the Roc wing from the next room. Go back and save your game then equip any DSS shield and head up the right hand wall of the tall room to find the hidden room. Grab the other extra item and head out the upper left door of this room. At this time, you can go back and grab any extra items that you may have missed or passed up. From here, head up one door to a warp room and warp to the area near Necromancer. Head out of the warp room and up through the ceiling. Head left one screen and head up through the ceiling of the next screen to reach the Observation Tower.