From Quake Wiki
Quake is a 3D world, be sure to look up and down as you explore a level. Underwater areas are sometimes the only way to proceed, so go swimming when you have an opportunity.
Weapons and Combat
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For tips on dealing with each of the monsters and bosses in Quake, see the Monsters page.
If you find yourself at 95 HP but find a health item, sometimes, the best course of action is to simply ignore the item for now and come back to it when you find yourself more wounded, as even the small heath item will only heal you 5 points instead of the ordinary 15 points if you were more badly injured.
It is also very important to manage your ammo in Quake. As such, some weapons are better left for specific enemies. For example, the Shambler has a 50% damage resistance against explosive weapons such as the Grenade Launcher or the Rocket Launcher, so the next most effective weapon to use would be the Super Nailgun or the Thunderbolt.
Quake rewards creative and explorative players with secrets. If you see an odd texture on the wall, chances are you can shoot it to unlock a secret pathway to an item, such as in E1M1: The Slipgate Complex.
Not all secrets are found by blasting buttons, though. For example,
Main article: Trick Jumps
Trick Jumps are maneuvers that allow you to achieve a speed, height, or distance beyond what is usually possible with the physics of Quake. In general these 'features' were not intended by the designers at id Software, though there is some evidence that at least some of them were known to the designers during development. They are almost never necessary to progress through a level (with the exception of a few rare user-made levels), but can often be used to take a shortcut to another part of a level.