Marathon

The Marathon series is considered by many to be a trilogy of cult classics. They are also somewhat of a black sheep among the well known retro FPS titles. It was the first shooter to have mouselook, room-over-room level geometry, friendly NPCs that help you fight enemies, its tied with ROTT being the first FPS with akimbo weapons and levels specifically for multiplayer. Its also an early game to use alternate fire modes and mission objectives. For 1994 its really impressive and ambitious.
The visuals are nice and the music is mostly good. The game has plenty of story, which are presented by text screens. Its about an AI named Durandal that went rogue. He along with the other AI, Leila, tell you what's going on, what your mission objectives are, and will show you where the level exits are in some early parts. The mission objectives aren't complex, and a lot of missions don't even have any objective other than getting to the end of the level.

The level design at its best can be average but terrible at its worst. A lot of the levels feel like early Doom user maps, most of them are really amateurish and mediocre. The worst by far is this map where you need to make pillars rise to form stairs, but you have to go back and forth over and over to the different switches. That level also has a window where you can see weapons that are inaccessible with no secret entrance, just as a middle finger to the player. Its one of the several levels that makes the player ask "Why would you do it like that?". For example, why does the last level end so anticlimactically? Why do some large never before seen monsters show up in the second to last level instead of the last one? And why are there no health items?
The answer to that last question is that Marathon only has health stations. Unlike the ones seen in Half-Life, these have unlimited health. Some health stations can give you a double or triple health boost, so naturally the player wants to use health stations as much as possible since its crucial to not dying. Another station you can find is for saving progress. You can't save any time you want but the upside is you don't lose your weapons on death.

The combat in Marathon may be its worst aspect. Your stock sound effect punch attack is pretty weak, your handguns are slow pea shooters, the plasma pistol is only good when charged, and your main workhorse weapon is a liability. Since Marathon 1 does not have a shotgun, your primary weapon is this machine gun with poor accuracy and has an attached grenade launcher which is also a big letdown. There are some better weapons in the game but the real reason the combat is so bad is the reloading.

You cannot reload your weapon on command, you need to empty the magazine in order to do that. Its made worse by having very slow reload animations. You will constantly have to slowly reload during a battle and take damage. This is one of the handful of decisions that nearly ruined the game. Ammo can also be sparse so it might be best to play on the hardest difficulty which doesn't limit how much ammo you can hold. Turning down the difficulty level doesn't make Marathon that much less of a chore to play. Either way its frustrating whether it be from lack of ammo pickups or unforgiving combat.

You constantly go back and forth from health station to save station, while navigating clunky mazes and getting in firefights where your primary weapon makes you push forward because of the inaccurate spread. But being close makes you die or take damage because of the bad reloading mechanic so that's even more running to health and save stations in clunky mazes that feel like its just trying to waste your time. It is so fundamentally broken, it all works on paper but these things all bring the game down to a point that it doesn't matter how good the story is. The gameplay is anything but intelligent yet they clearly really cared about the lore and worldbuilding.

It makes sense why they wanted to give Doom a run for its money while ditching keycards, finite health items, and manual saves. However, trying to avoid the features that made Doom simple fun requires a lot more understanding about why Doom's gameplay works, which is why Marathon isn't all that fun at all. It takes few steps forward and many steps back.