This level contains 6 secrets and a super secret.
1/6 - Once you reach the upper part of the town, follow the narrow opening to the right of the central building to discover a rune alongside some heavy ammo. (Inferno Rocket Slaughter III)
2/6 - Following from the previous secret, get on the roof opposite the save point, turn around and jump-dash onto the burning beam to your left. Follow the beam around to reach the armor that can be seen on the adjacent balcony.
3/6 - Upon clearing the town center out, go into the opening to the right of the lift and use the highlighted wall at the end to reveal a hidden alcove.
(*) If you opt to dive into the huge crack at this point, you might as well check the super secret to avoid backtracking for it later.
4/6 - Taking the slope up from the center, look for a torch all by itself on the path to the left and use it to discover an armor on the other side of the wall.
5/6 - Following from the previous secret, destroy the cracked part of the mountain at the end of the same path and get teleported to the side area. Carve your way through and loot the stash found at the back of the building to the left of the teleporter.
6/6 - As soon as you are back from the side area, unlock the metal door to your left and use the door of the adjacent building. Grab the items on offer as needed and explore further to come upon... a skeleton with a ring on her finger. (Inferno Gauntles Expansion III)
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SUPER SECRET - The dead need to be wed once again. Dive into the underhalls via the crack next to the save point at the town center and look for a blue-lit torch in the immediate area. Use the window right next to it, get in and grab the flame suppression potion from the table.
Continue on to the gallows up the stairs beyond the yellow gate and use the potion on the skeleton second from the right. Once the flames are gone, get behind him to pick up his ring. Return to the other skeleton (secret #6), give her the ring and grab the goodies from the newly-revealed room next to the door. (Vampire -G- and a Blood Gem)
(*) You should be able to unlock the last rune slot by this point.
(!) The achievement gets unlocked the moment Templar states that he found a super secret.

This level contains 6 secrets and a super secret.
1/6 - Starting out, get to the other end of the hall you are in and take the ladder up. Follow the walkway, jump-dash to the ledge above the save point and use the wall next to the pitchfork and the bundles of dynamite to reveal some heavy ammo. 'Victims, aren't we all?'[blood-wiki.org]
2/6 - Once beyond the veil, cross over to the island, turn around and jump-dash to the ledge below to get the ammo and the health packs laying out in the open.
(*) It might be better to check the super secret at this point, as backtracking for it later would prove a hassle due to the linear nature of the level.
3/6 - Out of the mines, take the path to the right and destroy the cracked wall at the end to reach the teleporter to the side area that starts familiar enough.[dukenukem.fandom.com]After clearing up the place, go through the opening next to the central pillar to find an armor and a health pack around the corner.
4/6 - Before leaving the ruins behind, take the path on the right to its end, drop down to the ledge below and open the door to find a rune and an armor. (Inferno Gauntlets Slaughter III)
5/6 - Past the intermediary area after the crypts, use the wall with the unlit torch at the corner to reveal a hidden compartment with an armor and lots of heavy ammo.
6/6 - Before dropping down the hole that doubles as the exit, jump on the artery to your left and follow it all the way up to reach the rune residing on the ledge. (Inferno Revolver Slaughter III)
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SUPER SECRET - A curiously placed lantern needs to be lit. Stepping into the mines, go straight ahead till you reach a dead end. Once there, use the mound glowing purple on the right to get a flameball. As soon as you leave the mines behind, take the path to the right (secret #3), turn around and jump-dash to the path below where a lantern can barely be seen by the cavern wall. Use it so that it gets lit by the flameball and go through the newly-revealed opening to find the hidden cave of goodies. (A Blood Gem)
(*) Thanks to Schneekanonenschütze for shedding some light on this one - no pun intended.

This level contains 3 secrets.
1/3 - Missable. Right at the start, turn to your left and jump onto the artery at the corner. From there, utilize the save point and the upgrade panel as stepping stones to reach the thin ledge by the circular teeth on the cavern wall. Use the indented circle to reveal a hidden alcove.
2/3 - Missable. Upon reaching the second save point, turn around and jump-dash from the tooth to your right to the nearest spike across. From there, simply get onto the other spike to grab the armor and the heavy ammo on offer.
3/3 - Missable. Upon clearing out the central area, look for a throbbing thing on the backside of the wider artery next to the brain. Shoot it to open up the otherwise unreachable alcove at the end of the adjacent walkway that contains an armor.
(*) Note that there is no going back once you step into the red portal.

This level has no secrets.
So, a straightforward boss level with no secrets whatsoever... or is it? Actually, this is where the book pages you collect along the way fit in to finally get that campaign-wide 'mega' secret. For the time being; gear up, re-spec as needed and leave the cabin to pull a Caleb on your master.
(!) The achievement gets unlocked after Templar returns the favor.
After the ending cutscene, return to E5M4: The End via the level select menu. Instead of leaving the cabin, check the bookcase by your bed to find a book laying there all by itself and use it to open up a portal by the front door. Stepping into the portal loads up E5MX: After the End, in which you have to face an altogether different threat. Defeating that thing awards you the rune Bubble Gun -G-, which you are able to equip once you load any other level or start a NG/NG+ on the same difficulty level you finish the campaign.
If you are unable to open up the portal, that means you are missing one or more of the four pages you need to collect throughout the campaign. No worries though - those pages are handled the same as runes and blood gems, in that, they are tied to your profile the second you pick them up (yet again, only for the difficulty level you are on). That means you can go back via the level select menu, collect the missing page(s) and return to E5M4: The End once you are ready.
Here is a quick recap:
(*) For the record, by the end of your run you should have: all the rune slots unlocked, alongside two unused blood gems and a total of 118 runes in your possession. Yes, Inferno Rocket Reach 2 is nowhere to be found, even though you have the first and the third iteration of the same rune.

THE REMAINING ACHIEVEMENTS
If you still do not have these two by the end of your run, here is the perfect thing... Return to E1M1: The Beginning through level select and make your way to the first upgrade altar as usual. Re-spec if needed so that you are equipped with Templar Hourglass II, Pistol Slaughter III, Pistol Rush III and Neo. Considering you do not waste any along the way, you should have a full bar of SMT by the time you are about to step outside. There are exactly eight monsters waiting for you to come out, yet... 'there is no spoon.'
Guardian is a pretty solid bonus mode that can be launched from the main menu which sees Templar defending the crystal at the center of the arena against endless waves of enemies that get more difficult to manage by each round. Alongside all the mechanics and weapons from the campaign (minus Inferno Gauntlets), it has a few additions of its own -that are also explained on the grounds- to help Templar in his stand.
Most importantly though, it goes hand in hand with the campaign in terms of runes - i.e. what you unlock in the latter also becomes available for you to use in the former. Thus, to increase the chances of surviving for at least ten rounds, it is better to leave these two achievements aside until you get all the others (they do not stack either, so you have to do each run separately). Just remember that if things get too hectic, you can always bubble your way out.
CLEAN SLATE
If you ever need to start from scratch (at your own risk); first disable Steam's cloud saving for the game, then locate the DT folder found under User\AppData\LocalLow\T19\DreadTemplar and delete all the .dat files within. Doing so wipes out everything associated with your profile, be it settings, blood gems, runes, unlocked levels or high scores.
Once you get the game running again, minimize it and re-enable cloud saving. Upon quitting, Steam asks whether it should keep the files in the cloud or upload the new data - choose the latter to replace your outdated profile with the new one.

THE TEMPLAR LOOKS
Like the looks? I made a custom library set for the game by combining one of the hi-res artworks of Giovanni Fim[www.artstation.com]and the clean-cut logo uploaded by Zach Fett to SteamGridDB.[www.steamgriddb.com]The package of four images is available here[drive.google.com]for anyone to use.
THE TEMPLAR RUNES
Check out FrozenFirebug's guide for in-depth information on the golden runes:
...and here is my preferred build - I do not use Tri-Rocket Launcher or Inferno Gauntlets R at all:
THE TEMPLAR TUNES
Stream and purchase John S. Weekley's soundtrack:
Dread Templar (Original Game Soundtrack)[johnsweekley.bandcamp.com]
THE TEMPLAR CHRONICLES
Last but not least, have your fifteen minutes of Templar fun in glorious 2-bit:
Hellhunt GB -1991-[t19games.itch.io]
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'After this final battle, my rest is well deserved.'
These guides are up to date as of version 1.0.2b of Dread Templar and considered 'complete'. Feel free to comment under the relevant one if you require further assistance.