Hexen/Silent Refectory

A "refectory" is an old term for the kitchens and dining areas of castles, monasteries, etc. Three of the five planet gems you need are in this level. You enter the level in a large storage room packed with crates. First, make your way around the crates and clear out all of the enemies. You'll find a few crates with a symbol on them. On the wall nearby each will be a switch that opens a door or passage in the immediate area.

Emerald Planet and exit
Near one end of the storage room is a small side area. Look for and pull the switch at this end of the storage room: a stairway inside the side area will slowly lower. At the bottom is the portal returning to Heresiarch's Seminary. On either side of the portal are small networks of passages to a room farther back, with a terrace around the back walls. Pull the switch on the terrace: a door opposite the room opens, and steps to it form. Inside is an Emerald Planet.

Tall circular room
From the side area and stairway, follow the right-hand wall of the storage room until you come to a stack of crates with symbols. Find and pull the switch on the wall to the right, and the crates--a hidden lift--lower. There's a nasty Slaughtaur ambush at the bottom and no cover, so have weapons ready. (Flechettes are especially useful here: for the Cleric or Mage, set off gas clouds or bombs down inside, and for the Fighter, stand at the top of the lift and throw them down the hole.) At the top of the stairway circling along the wall, you'll find a switch. Get weapons ready, then pull it. You'll be accosted by Dark Bishops that spawn in. (This is a good place to try out your new #4 weapon if you haven't yet.)

Dining room
Back in the storage room, go to the opposite end from the side area, and look for the switch near the crates with symbols. Pull it to open a hidden door nearby. Inside is a small ante-room, and past here, the dining room. In here you'll find a wooden door to your left, and a doorway open at the opposite end off to the right. This was a hidden door that the switch in the circular room opened. Go in here, and follow the stairway around and up. This is the area you saw through the windows in the circular room. Pull the switch at the upper end of the passage, and return to the dining room.

Sapphire Planet
A second door to your right opened when you pulled the switch. In here is a large, round deep-looking room with a big column. When you step forward, the column begins opening in layers. A group of Dark Bishops down below will notice you. Most of the floor in the deep part will rise up to your level. When things finish shifting, there will be a small pillar, on which is a Sapphire Planet, surrounded by a pit. It's deep enough to cause some falling damage but won't be fatal unless you're very low on health.

Jump or run onto the pillar and collect the planet gem. When you do, the pillar will momentarily lower to the bottom of the pit. (This is your way out if you fall in.) Try to skid to a stop on the pillar to avoid falling.

Before continuing, there's a hidden bonus area in the dining room. Look along the walls for a purple mark above some artwork. (If you came in here before opening the two hidden doors, those doors also had the purple marks. The one you want now is a third one in addition to those.) Push against the wall, and it will slide back. Push it all the way back, and you'll find a hidden compartment on your right. In here is a Banishment Device.

Ruby Planet
Go to the wooden door in the dining room. In here are two more doors to either side. Both lead to mirror-image rooms, with nooks on the walls and a pool of either water (right) or lava (left) taking up most of the room. In both rooms are revolving doors (with the silver kick-panels) that you manually operate, i.e. push on them to spin them around. Both lead to a small central room with two windows looking into dark side areas with enemies inside. At the end is a Ruby Planet. When you pick it up, be on guard: the two windows convert into doorways, and the back walls open and release additional enemies waiting inside.

In the room with lava, one of the nooks will have a tunnel now opened inside. Follow this and activate the wall at the end to open a hidden door back to the storage room. Once you have the three planet gems, you're done here.