Hexen is based on the Doom engine and uses (mostly) the same tools, but adds a few more features to the mix. (G)ZDoom actually used the Hexen map format even for Doom maps (Doom in Hexen Format) because it was more capable, before eventually switching to UDMF which is even better. Polyobjects are a hacky way of doing 3d-ish moving geometry created for Hexen. It can be seen in the first map, with a pair of towering wooden doors that swing open ominously. (G)ZDoom extended them quite a bit and that's how the sliding and swinging doors are done here.
The final boss is (barely) killable with the ammo in the room. Old shooters didn't have autosaves, but there's a critical quality of life feature these days. On top of that, some of the more complex (G)ZDoom mods would break horribly with an oldschool pistol start.