I strongly suggest that you do the Towers in the order of middle, left, right.
Because of the reference to yellow light, I assume you're in the middle tower. That yellow light relates to something in a different part of the tower. To get the hourglass
throw the switch in the top part of this map. Go down to the bottom, left and first up. Throw the switch. You can down access another switch by going back down, all the way left, up to the top, right and down. Throw that switch. Now fight the monsters. Return to the second switch and throw it to lower the spikes and get out. Can't remember at what point the spikes go down so that you can get to a golden bowl near the 3rd switch with an item in it. You'll have to just keep strolling along there and check.
When you're done here, return to near where you started and go east to the next screen. That yellow light released the spikes behind the O'hr on the path above you. Behind that monster is the hour glass.