You know game level probably was vertical slice? Doom E1M1. It basically has everything you know about the game: it's first person; you can attack monsters with your choice of fists or guns; your ammunition, armor, and health are limited; you can pick up ammo, armor, health, and additional weapons; your enemies can attack you with their own melee attacks or with hitscan or projectile weapons; the maps have areas that can harm you or have secret doors that can contain secret caches or routes; there's objects that can harm you or your enemies; and there's an defined exit. The gameplay evolves later on, with enemies having higher health, being faster, flying, invisible or using some variant form of those three attack vectors (or, in the case of Doom II's the Archvile, the ability to resurrect enemies); your own weapons building on those same three attack types with different effects (for example, the rocket launcher, plasma gun, and BFG are all projectile weapons, but differ by having splash damage or generating its own hitscan attacks); power-ups granting invisibility, berserker strength, or invincibility. But iD had to walk before running with those elements and you can see it with E1M1.
