Oh we definitely try "simple" approaches like that, whenever we can. And if the entire 'level' is, well, level & there are no elevation changes (or elevation c

Oh we definitely try "simple" approaches like that, whenever we can. And if the entire 'level' is, well, level & there are no elevation changes (or elevation changes are restricted to '1 elevation per floor of a structure, for instance) then sure, that type of solution works. Otherwise though, the 'trigger' code you suggest ends up generally being some kind of math (ie physics, regardless of whether it's via a physics engine layer or a game/state layer); you essentially have to figure out "where" on the plane the object is, convert that to the proper coordinate system for your overall level, and then 'sample' the underlying terrain/mesh/ground covering to figure out if you're in a legal position. Typically this ends up being implemented as a "clamp" of sorts; you will pretty rapidly end up with oddities, floating point precision quirks, and the like. The "best solution" will vary, often drastically, from game to game. FWIW, I've been doing game-dev for almost 10 years now & have almost exclusively done "AI/NPC" agents. We've faced what I would classify as pretty much the same issue with AI agents, especially when you want them to follow nice, smooth paths while also realistically 'bumping into' each-other -- yet ensuring that they will never, ever, ever go through a wall. It sounds so stupidly-simple but, I can assure you, it is not! If you had unlimited resources (read: CPU), then it's not so bad, you can calculate for "optimal" every frame -- in reality, you never have unlimited resources, and seemingly-simple calculations like this have to run extremely fast and they cannot be allowed to fail (there's nothing more frustrating & game-breaking than NPCs that get 'stuck' behind a wall or in an area that they cannot vacate, etc). You can write code for agents to "detect stuck + get un-stuck" but, similarly, doing that fast is non-trivial; and ideally you don't want them stuck to begin with!! It's a massive can of worms. ;)