Monsters on slopes

How good are monsters at navigating slopes? Any typical issues to watch out for? Are they sometimes unable to come down, just like with steep stairs?

In my experience, about as good as navigating normally. I didn't test their ability to move up and down the slopes though, and I doubt they're capable of doing so.

Depends on the slope. If it's steep enough, the monster won't try to come down, though like with 3D floors, it could fall down, regardless.

Shallower slopes can be navigated normally as if they're standard floors.

The kinds of slants monsters cannot navigate are the ones of 50-ish degree elevation or steeper. You can test it for yourself - if you're sliding down the slope while not moving at all by any inputs, monsters won't be climbing/descending your structure either.

IIRC in ZDoom the cut-off point is 45°. Less than that and monsters can navigate it without problem, more than that and they can't.

The general rule for monster downward movement is: A monster cannot step onto a position where the monster's hitbox would overlap with any sector that has floor height more than 24 units below the monster's feet. In the process of checking this, the height of a sloped floor is calculated as the height at the lowest point on the sloped floor below the monster's hitbox. The monster's feet, on the other hand, cannot be lower than the height at the highest point on the sloped floor below the monster's hitbox.

Therefore, for a monster to climb a slope, the slope must not be steeper than <the monster's diameter> horizontally per 24 units vertically. Even less steep (precisely, up to square-root-of-two times less steep) if the slope doesn't go directly north or south or east or west. In the context of monster movement at least, a slope could be considered practically equivalent to a staircase with infinitely thin/low steps.

@scifista42 actually explained why my backstage experiment trying to prove @Gez's (albeit "IIRC") point failed.

The thing I quickly threw together is E1M9 in this wad. (UD, ZDoom-compatible)

Technical background: the initial brimstone floor is at 0 height, the three "prongs of the fork" are 31, 32 and 33 each, and they all elevate by 32 pixels horizontally, thus rendering the slope of the middle prong exactly 45° steep (at least by ingame mechanics), whilst the left one is a bit less and the right one is a bit more slanting. The lines separating the soil from the brimstone are tagged "block monster" each so the Imps I use for the experiment wouldn't escape and make things harder.

Turned out they have too small hitboxes for the "slope block" taking effect, they all "slided" both up and down on them with ease. Perhaps I should either make the portrusion higher or replace them with wider monsters.