Block only grounded monsters?

Do you want to also block the player, or not?

The classical way is to create a thin crevice of at least 25 map unit depth so that walking monsters won't cross. You can hide the lines in the automap and use Boom height transfers to make the effect invisible to the player. Players (and monsters that ignore dropoffs, like MBF dogs or Heretic's maulotaurs; but in vanilla Doom they don't exist) will not be stopped, and a dead monster's corpse can slide across too; it's only an obstacle to the AI.

In ZDoom, you can also use the "Strife railing" flag, which creates a 32-unit height block. Walking monsters will be blocked, as will players -- unless they jump above, or they get a 8-units-or-more step up from a nearby detail. ZDoom also has a "block creatures" flag that affects walking actors, monsters and players alike, but lets floating actors go across.

Only monsters, so it seems I need to go with the first method you mentioned. I'm working in UDMF, which I should probably have specified in the OP (might convert to Boom compatible in the future, but that will be another issue another day...), but I assume you can do exactly the same thing in UDMF as well?

Yes of course.

4 hours ago, Malefication said:

... (might convert to Boom compatible in the future, but that will be another issue another day...) ...

Interesting idea. Usually it's the other way around.

Anyways, if you want pursue the conversion then be aware that there is no program which would convert any linespecials from UDMF to BOOM properly. You have to accomplish that manually.

Yeah, I'm aware of that. I picked up UDMF as the first (and only) format when learning to map again after a long break. I'm not using any extreme ZDoom features, so while converting any maps in the future will suck, it won't suck much more than editing loads of linedefs. Hopefully.