WHAT'S BEHIND THE DOOR
TR5 Level by Charles Kane
Walkthrough by Treeble
Once the level begins, head to the left to spot a door in the central niche. That's your end goal, it's flanked by a keylock and shelves you can search for a whole lot of WEAPONRY and medipacks. Turn around and throw that switch to open the gate, run down the corridor for the next switch and proceed to the next area.
Notice the golden rose behind bars, we'll get it shortly. For now, hop over the block and push the smaller shelves all the way onto the tile in the back, then search the nearby shelves for more supplies before climbing the blocks to the right. Hop across the gap and time your strides past the battering rams.
Follow the pathway until you arrive at a room with a flashing floor. If it's not obvious enough, those are deadly tiles, so play it safe and jump to the block in the middle of the room to claim the MYSTERIOUS DOOR KEY. Then tread the safe path to the back corner where you can get a golden rose (SECRET #1), as well as throw a switch in there to unlock the rose you've seen earlier.
Retrace your steps, making a quick stop for that golden rose (SECRET #2) before unlocking the door in the initial area. To answer the titular question, you'll find no less than five hydras within. If you're set ablaze, jump or crouch so that the ankle-deep water will douse the fire off. Best strategy to deal with these guys is using the shotgun and shooting them at the mouth when they start to charge up a fireball, so keep tapping the look key to shift around targets until you have the right one and best of luck. Once all five are done for, retrieve the final golden rose (SECRET #3) in the bright yellow spot on the right hand side of the room and go throw that switch to wrap things up.