Haunted Halls: Green Hills Sanitarium is a hidden object game with some elements borrowed from standard point-and-click adventure titles. The player assumes the role of the female protagonist, a woman trying to find her boyfriend after he disappeared while investigating an abandoned asylum. The main objective is to collect a series of objects and use them to solve all the puzzles found on locations related to the plot. The items are stored inside inventory slots at the bottom, from where they have to be dragged and dropped on interactive portions of the scenery to perform an action or complete a task.
Some of the necessary tools are received after completing one of the hidden object hunts, available on certain hotspots shimmering with red sparkles appearing on several locations. They take place on separate screens, where all the items from a list at the bottom have to be found amongst the assorted paraphernalia cluttering the scene. The hint button is a jar with arcing electric nodes at the bottom-right corner, showing the position of one of the required objects when activated, but taking some time to recharge after use.
Places with green sparkles have one of the mini-games, taking place on separate screens where mechanisms of some sort have to manipulated to solve them. Several puzzle variations are featured, where the player has to rotate circular sections of a picture to unscramble the image, rotate wires to complete a circuit and finish other similar challenges. They can be optionally skipped after a few minutes.