A Garden Maze

Thinking about a dungeon, I was struck by the thought that they would work similar to above ground mazes. Hell, Cursed Chateau from LotFP mainly features one surrounding the titular house. You could have the party navigate from the stars, threatening them with wilderness hazards like rain and cold, unlike the standard climate-controlled dungeon with its solid roof.

However three problems present themselves: The party can go up and over, through the hedges or tunnel beneath.

Restricting player movement is key in a dungeon as it allows for the party to not just wander straight to the biggest collection of treasure and to keep them from wandering into an encounter you don't want them to. But hedges are substantially weaker than stone walls.

The solution is entirely up in the air, the players could be barred from above by charms (in that case why not have a roof and set it underground?) or flocks of angry birds. Or could simply be surrounded by fog, but that's not a fair answer.

Going through sounds like a perfect excuse for a monster however; animated topiary, poisonous thorns and killer squirrels might lurk in the hedges, with each having been statted during AD&D's long history. Another alternative is that the hedges are all connected in a sort of continuous state, and so entering could teleport a character to a 10' wall anywhere else in the maze. Once the party figures it out it could make a last-ditch escape attempt if things go bad. And thorns in general could dissuade low-level characters.

Going under, is actually the easiest to deal with. Not only does it take a while to dig, but every turn spent digging, attracts a random wandering monster to the location or the opposite location on the other side of the hedge, if the party are past the half-way point of the dig.

Not even the soft soil dig speed of a dwarf could complete a tunnel before attracting monsters. And if the succeed? We'll good for them, now they have a tunnel between two points and need to start mapping again.

Even so a maze has its limits, once a party get access to fire spells (a torch and oil won't ignite a hedge unless its dry, otherwise it just smokes like a beacon) the maze is so much kindling, but so are dungeons, once the MU gets access to Stone to Mud.