Level Boundaries

That's another thing that's hard: limiting the space the player can move in. It's easy if the level is indoors, all corridors, like Nova Prospekt or the likes: just lock the doors. Nature's limit on level size.

But outdoors? It's hard to put impassable barriers anywhere without making the gameplay feel stilted, awkward, forced. Artificial.

You're in a part of the city where every exit has conveniently been blocked by collapsed buildings? That doesn't make sense, then how did you get in there, and how is Gordon gonna get out there once the screen fades to black?

Or put up traffic cones and riot barriers and cover them in toolsinvis? That's even worse. Gordon can shoot down helicopters, but a traffic cone is enough to stop him? Nah.

Separating the playable environment from the backdrop that's just there to liven up the scenery is pretty hard, at least in my experience. Guiding the player naturally, so they try to go down routes they can't as little as possible and can tell easily where they are supposed to be and where not is even harder. Campaign maps don't have this problem; each level transitions into the next, so the questions of entrance and exit are already answered, and having only the exit ramps blocked while the highway remains open is much more plausible than walling the player in on all sides. But standalone maps have to have a beginning and end that makes sense (at least that's what I want for mine), and that's tough.

I haven't really found a good answer for it yet. Entrances are easier than exits – just some drops that the player can't climb back up –, but it's something I struggle with immensely (and the way my brain works, I can't get to work on the rest of the map until I've solved that problem – it sucks, really).