Another technique heavily used in Tempus Irae is the use of "Rear-Admirals". This is when you enter an area - and are surprised to be attacked from behind. Sometimes, you're not surprised at all- but the option to turn around and run has been taken away from you.
So, in whatever manner you see fit - eliminate the trooper, hunters, enforcer and fighters in the start of the level. Make your way inside the building - where you'll stumble across some shackles. Before long you'll again be under heavy enemy fire - clear the area. Venture down the hall to the terminal, and a small ammo stash. You'll also find a 1x power up - but it is suggested to save this until you desperately need it - which you may.
The player must stand atop the first gallows to open the downward spiral staircase. Down here are a series of columns, and a shallow sewage pool. The smallest of these is blocked off from entry (sort of), unless one grenade hops over the retaining wall, or sidestep/leaps from the top of the stone staircase over the small wall. There is yet a third way - and my personal favorite: a hallway separates the 2 pools, one side has stairs - the other a wall. Go down the sewage path with the stairs, to the end of the "runway"...turn around - and sidestep/run full speed towards the stairs - as you escalate up the stairs - turn left (while sidestepping/running left), and sail with ease over the wall...well, with a little practice you can do it with ease ; ) By accessing this small pool, the player is teleported to a secret room which contains an ammo stash and a powerup, as well as a very revealing secret terminal. This terminal explains how to gain access to several of the secrets in Tempus Irae. A teleporter takes you back to the stone staircase. Activating the switch at the door opens the door directly across the hall.
Take out the trooper, drones, enforcer, fighters, trooper #2, and the hunter - stumble across a Leo book (which you can't read), a terminal, and a switch. The switch not only opens the door allowing you to leave, it also activates a secret platform in the area where the enforcer was. This small platform leads to an AR clip - and a broken teleporter (a prelude of things to come?). Exiting this room opens the barrier which unveils the second half of the level.
By hopping on the chair in the small library, it activates a 1x powerup. You may need to save this. The switch in the library opens the door in front of the fireplace. The door in the middle of the hall is full of aliens - decorated in every color. Eliminate them all. In the back of this large pillared room is a switch - it unveils an underground passage.
The room with the half opened door (with the concealed 3x's powerup) is the secret exit.
The room with the manuscript contains a secret room behind one of the bookcases. It can easily be accessed by standing on the table and tabbing the wall in the North East corner of the room (to the left of the terminal hallway). Inside you'll find more ammo and a 2 x's powerup. You must access the terminal in this second library (manuscript room) to activate the switch for the room down the hall. A switch at the top of the mini spiral staircase opens a door to use as a shortcut for re-accessing the save term (across from the fireplace), or for retrieving that "saved" 1x powerup.
Once you have read the terminal to activate the switch for the next room, be sure to re-load your weapons...the next room is Hellish. Inside you'll find more than enough fighters, a MOAH's, a MOAC's, some troopers - a 2x's powerup, and a ton of ammo. After clearing the room, go up the stairs and exit through the door atop the fighter perch.
Outside (on top of the second gallows) is another ammo stash, and 2x's powerup - missile, or grenade hopping can get you up there.
The next indoor area leads to an (apparent) exit teleporter - but it's broken, so you'll have to find another way out. Once you've activated the switch by the buildings exit, you can proceed through the outdoor courtyard and go inside the building facing the first gallows. These caverns lead to an inaccessible ammo stash, a 3x's powerup, and a deep crevice - which leads to the exit teleporter.
The other option is to leave the building with the broken teleporter, and instead of going through the courtyards - go back to the room with the (previously) half opened door (and *now* accessible 3x's powerup) - it's now open. This teleporter will take the player to the secret level The Game Of Death.