After the first time you've become the Dark Prince and changed back to the Light Prince. You'll go through a hallway with several beams of light coming down and a bunch of enemies. After using the wall mechanism to open a door, go through the door to find traps to the right and a doorway covered by grey curtains to the left. Go through the curtain covered doorway to the left. Continue to the end of this hallway to find the Life Upgrade Fountain.

Previous Save Point: The Lower City Rooftops
Next Save Point: The Arena

After the short cut-scene showing you getting saved by someone shooting arrows, continue across the rooftops and down to ground level. When you get to an area with picnic tables, use the wall mechanism to open two doors, one right next to you and another further away behind you. Go through the door further away and behind you and continue through the passage to find the Life Upgrade Fountain.

Previous Save Point: The Temple
Next Save Point: The Marketplace

After playing through the Dark Prince portion of the area you'll enter a room filled with water and return to being the Light Prince. Drain the water here and as you start ascending through the room you should notice an opening covered by a banner/flag. Jump through here to find the passage to the Life Upgrade Fountain.

Previous Save Point: The Canal
Next Save Point: The Promenade

If you are facing the Save Fountain for The Canal, turn right to find a small opening at floor level. Roll through here to find the passage leading to the Life Upgrade Fountain.

Previous Save Point: The Palace Entrance
Next Save Point: The Hanging Gardens

In the "Lift area". (where Farah enters the Lift and you have to find a way to activate it). As you make your way up the room you will encounter levers that open doors. At the second lever (guarded by some "female" sand-creatures) there is a row of "dagger-holds" on a column. Use them to climb up and you should see a switch. Rebound jump a couple of times on the wall and you should be able to reach it. once you do, immediately slow time, let yourself drop to the second"dagger-hold" and use it to go back to the lever area. The second door (the one the lever does not open) should now be open just long enough for you to enter.

Previous Save Point: The Middle Tower
Next Save Point: The Upper Tower

This one is only easy to miss if you aren’t paying attention due to how late in the game it is. I am surprised I missed it at all my first play through. After you get your father’s sword (sword of light) you will eventually start to climb outside of the tower of Babel. There are occasionally large wooden constructions that you get to move a brick about in order to move on. 3 I think total. You want to pay attention on the second one. Once you get to the top of the wooden outcropping it will give you the icon thing to let you know you can get a distant camera look around. Use it and you might notice an odd (cracked looking) doorway, in first person the crack is not visible so that’s a clue. Finish this wooden puzzle and you will find yourself over it. One more jump forward gets you back in the tower, instead look below for a bit of wood jutting out that you can drop to. Do so and you should find the last health upgrade.