Heretic II sequence breaks
While Heretic II tends to be rather linear, there are a few places where you can break the intended progression by exploiting oversights in the level design. Beware that due to the linear progression of weapon and spell placements, which the designers intended to be unmissable when you follow the "correct" route, you risk missing out on certain items for the entirety of your run if you do some of these sequence breaks.
I haven't yet explored the whole game with a mind to finding these, so for now the ones I know about are mostly for the early section of the game. If you know some others feel free to submit some.
To pull these off, a useful technique to master is the wallkick/walljump. Face a wall and jump at it, and then, when you are in contact with the wall, hit "move backward" and "jump" simultaneously. If you do this correctly, you'll kick off of the wall and gain a little extra jump height. The Heretic II map designers seem not to have taken this into account in some places...
You can use the wallkick to get up to the balcony where the wizard is, near where you get the tavern key. From the crate pile, jump at the protruding turret next to the balcony and kick off of the side of it. Doing this successfully lets you skip the tavern section completely and puts you immediately before the point where you meet Dranor. You will permanently miss the Ring of Repulsion if you skip this section!
From the start of the level follow the passage until you come out to the street and turn right. You'll find a blocked door and some barrels with an overhanging platform above. Face the door, jump up and wallkick to the left of it, then immediately spin around 180 degrees so you can ledge grab onto the platform and pull yourself up. Mouse controls help here. You'll be near a mana shrine and can continue further into a later part of the level, just past the big warehouse room with the crane and right near the button that you have to hit to break open the door you were just near. Beware that you can miss the Hellstaff if you don't go collect it before doing this sequence break, and won't have another opportunity to get one later.