This level contains 9 secrets and a super secret.
1/9 - Use the backside of the tomb near the upgrade altar you come across shortly to reveal a compartment that hides some ammo.
2/9 - Follow the purple torches to reach the corresponding gate on the left. Once there, make your way up via the rocky steps to the left, jump-dash across to grab the rune as well as the armor laying out in the open and descend into the catacombs via the gate. (Pistol Rush II)
3/9 - Arriving at the circular hall in the catacombs, use the statue with torches on its sides to reveal a hidden compartment that holds an armor and two health packs.
4/9 - Battle it out to the other side of the level till you reach the graveyard. Shoot the skull plate behind the tomb to open up the grave right in front of it. (Shotgun Slaughter II)
(*) It might be better to check the super secret at this point, rather than backtracking for it later.
5/9 - Right after falling down through the tomb opened up via the switch in the graveyard, look for an underwater opening and follow it to reach a hidden compartment with health packs and SMT energy inside.
6/9 - Following from the previous secret, jump-dash to the ledge holding a coffin in the next room, destroy the pottery and use the indented part of the wall to reveal another hidden compartment.
7/9 - Before going through the final gate, check the opening below the slope that leads to the said gate and shoot the cracked wall to reach the side area. Upon reaching the ruined section, check behind the coffins to find an armor and some ammo.
8/9 - Following from the previous secret, use the tomb at the next floor to find more ammo and SMT energy in the alcove behind it.
9/9 - Following from the previous secret, crawl behind the coffins at the corner of the same floor to find a skull plate. Use it to reveal a compartment with a rune and two health packs inside (Inferno Rocket Reach). The ammo on the other side of the fence can be reached via the opening on the ceiling of the first floor.
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SUPER SECRET - Follow the path to the left of the lever found in the graveyard to come upon a familiar looking mausoleum.[blood-wiki.org]Use the tomb with the candles on top, get inside and close the lid on yourself to get dropped down into a hidden room. Do not forget to check the usual suspects to see if everyone's favourite undead[blood-wiki.org]left anything behind for you.
(I Live... AGAIN! -G- and a Blood Gem)
(!) The achievement gets unlocked the moment Templar states that he found a super secret.

This level contains 10 secrets and a super secret.
1/10 - After the initial drop, destroy the crates stacked behind the arch to the left and use the wall behind them to reveal a hidden compartment with some ammo and two health packs, as well as a teleporter to return back to the starting point if need be.
2/10 - Upon reaching the closed gate, hop onto the broken ledges to the left of the overlook, turn around and dash onto the next one to reach the ammo laying out in the open.
3/10 - Shortly after the upgrade altar, drop down to the ledge below the path you are on and grab the hidden armor.
4/10 - Before stepping onto the hover plate, dive into the lake and swim into the tunnel past the upgrade altar to find the cracked wall that leads to the side area. Get in, clean out the place and use the wall at the end of the short tunnel to the right of the rune to uncover a cache.
(*) Be sure to pull the lever found within this area, as it lowers the iron bars in front of the other cave in the lake that also holds a Blood Gem (and R.P.D.'s once finest).[residentevil.fandom.com]
5/10 - Right after the first trip over the lake, check under the stairs in the following room to find hidden ammo.
6/10 - In the room with the lever that activates another hover plate for traversing above the lake, destroy the pottery and the coffin and crawl into the opening behind them to find a hidden compartment. By the way... 'what is the most resilient parasite?'[www.imdb.com]
7/10 - After activating the third hover plate, return to the previous lever and pull it twice more to rotate the walkway in the center of the room so that it connects to the wall panel with a skull plate on it. Use the said panel to reveal another hidden compartment. (Black Bow Slaughter II)
(*) Buggy since EA: keep tapping the use key while taking a few steps back in-between the tries, until it finally opens.
8/10 - In the temple beyond the purple gate, of the two coffins at the corner of the second room, use the undecorated one to find some ammo and two health packs.
9/10 - Following from the previous secret, use the wall with a blood trail leading to it at the corner of the next hall to access a hidden area. While in there, look for a panel on the ceiling that drips blood and shoot it to get... a bloody eyeball.
10/10 - Now that you have the first eyeball at hand, backtrack to the temple entrance and turn right to come across an altar in the open. Get behind the said altar and drop down to the ledge below to find hidden ammo.
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SUPER SECRET - Following from the previous secret, use the altar itself to reveal the other bloody eyeball. With the pair in possession (look for the first in secret #9 if you miss it), a path is revealed that leads to the ledge above. Take it to reach the items laying out in the open. (Powder Kegs Are Not Toys -G-, a Blood Gem and early Inferno Gauntlets -beyond the red teleporter-)
After grabbing the goodies, return to the area from secret #3 and check around the corner to find a now glowing emblem on an indented part. Use it, get in and pick up the book page found in the tiny library behind, which serves as the first part of the campaign-wide 'mega' secret.
(!) The achievement gets unlocked the moment Templar gets hold of both of the eyeballs.

This level contains 11 secrets and a super secret.
1/11 - Right after the first corner, follow the ledge to the left along the abyss to find a hidden area with an armor and some ammo.
2/11 - Once inside the fort, grab the not-so-hidden SMT energy next to the tank found to the left of the entrance.
3/11 - Following from the previous secret, destroy the coffin behind the tank on the opposite corner and use to wall behind it to reveal a hidden compartment.
4/11 - Following from the previous secret, this time dive into the slime contained within the tank in the middle to discover another hidden compartment. (Trap Launcher Mastery II)
(*) Be sure to go into slow motion before diving in to minimize the damage caused by the slime.
5/11 - As you traverse the fourth tank, drop down onto the slime to grab the ammo under the walkway.
6/11 - Following from the previous secret, destroy the crates next to rune found at the end of the tunnel and use the wall behind them to find some ammo and two health packs.
7/11 - Right after passing the purple gate, find the part with the misaligned texture on the outer side of the left pillar of the structure and use it to reveal some ammo.
8/11 - Following from the previous secret, dive into the empty tank found around the next corner and shoot the cracked wall to reach the side area. Take the middle path and use the off-color part of the wall next to the skeleton torso at the center of the slime pit (shown open in the fourth shot above) to reveal a hidden alcove with a rune and two health packs. (Inferno Revolver Rush II)
9/11 - Back on the main path, destroy the crates next to the stairs that lead to the yellow keycard courtyard to find some ammo in an alcove.
10/11 - Right after using the yellow keycard, dash across to the demon plate above the pillar with the help of the newly-raised grating and use it to reveal a hidden compartment.
11/11 - In the hall leading to the exit, jump on top of the crates stacked at the back corner and jump-dash to the ledge on the pillar. Use the part with different texturing to reveal yet another hidden compartment.
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SUPER SECRET - Return to the second courtyard via the ladder in the exit hall. There are a total of five skeletons to be found within the fort grounds. Look around until you find the metallic arm laying next to one of them, as shown in the first shot above (it changes places with every map load). Take it and go to the mass grave between the two courtyards. Find the armless skeleton in the said grave and give the metallic arm to him. In thanks, he opens up a previously hidden room nearby. So get in and grab the goodies. (Four-Leaf Clover -G- and a Blood Gem)

This level contains 10 secrets and a super secret.
1/10 - Right at the beginning, use the wall below the unlit torch to your right to reveal a hidden compartment with some SMT energy inside.
2/10 - After the opening battle, drop down into the central pit and use the part of the wall with the misaligned texture across the ladder to reveal another hidden compartment that holds an assorment of ammo.
3/10 - Following from the previous secret, dive into the slime found to the left of central save point and swim through to reach a hidden room where an armor and two health packs reside.
4/10 - Following from the previous secret, get to the second floor walkway via the ladder found inside the centerpiece, go around and jump-dash to the ledge above the entryway with help from one of the narrower ledges around the pillars on both sides to get the armor and ammo laying out in the open. Landing on the said ledge also opens up a way back to the starting area if need be.
5/10 - Up next, shoot the cracked part of the wall to the right of the central save point to find the teleporter to the side area. Continue on and use the wall to the left of the skeleton seen right after the section with hazardous fumes to reveal a hidden room. Pull up the levers found within the said room to disable the traps in the vicinity. Apparently 007[jamesbond.fandom.com]could not think of humping the walls.
6/10 - Back on the main path, take the left ladder out of the pit and battle it out to the purple keycard. After picking it up, use the wall right behind it to find some ammo.
7/10 - Drop down back to the pit, take the other ladder out and carve your way to the red keycard. In the room just before it, jump on top of the crates, shoot the cracked part that can be seen to the left and get into the opening to find some ammo and two health packs.
(*) It might be better to check the super secret at this point, rather than backtracking for it later.
8/10 - As you follow the circular path that leads to the final arena, get into the first room on the left and use the part of the wall at the corner with different texturing to reveal a hidden area with some ammo and two health packs.
9/10 - Following from the previous secret, move into the room that houses the lever for accessing the arena and use the outer side of the duct on the right to find a hidden armor inside.
10/10 - Following from the previous secret, look for a ledge below the bridge in the next room and drop down onto it to get a health pack.
(*) Note that there is no going back once you step into the red teleporter above the arena.
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SUPER SECRET - There is a bloodfall in the same room with the purple keycard. Stand under it till Templar says 'My clothes are now soaked with blood'. Now, there is also a shrine of sorts with skeletons hanging from both sides in the same room with the red keycard, next to where secret #7 is located. Approach the said shrine with your bloody attire on to make the wall behind the central bloodfall slide down, before you run out of time to do so. (i.e. 'My clothes has now dried up.') Go through the newly-revealed passage to reach another familiar looking area.[quakewiki.org]
id Software salutes you. (Bloodthirsty -G- and a Blood Gem)

This level contains 3 secrets.
1/3 - Missable. Before stepping into the one-way portal at the beginning, drop down to the lower floor and use the pillar next to the ladder to reveal a hidden compartment. (Pistol Expansion II)
2/3 - After dealing with the welcoming assault, look for an island to the back of the purple obelisk where two health packs reside.
(*) These health packs are not available on Nightmare difficulty. The secret provides practically nothing while playing on the said difficulty level.
3/3 - At the base of the red obelisk, use the slightly indented part of the wall between the two torches to reveal another compartment with an armor and a health pack inside.
(*) Note that there is no going back once you choose to face the boss.
(!) The achievement gets unlocked after Templar defeats the boss.
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Keep on exploring in the third episode: