Manaslu circuit is relatively less difficult to trek than Annapurna Circuit Trek and Everest Base Camp Trek. Yet, it crosses a 5106m high pass called Larkya Pass. Larkya Pass lies between Dharmasala and Bhimtang, and is certainly nowhere more than 5000 m. Hence, Larkya Pass might be quite difficult to cross away.
Basically, the level of difficulty of an adventure tour depends upon several aspects. As if you can assume whether a trek is hard or not by considering the altitude level, weather and climate, remoteness, available facilities, and so on.
Certainly, Manaslu is a rugged and desolate area, and trekking to Manaslu is rather to explore the comical cultures and beauty of Manaslu than a tourist visit.
Sometimes, you may feel rather harsh, depending on how you adjust to the lack of Oxygen. So you should respect this trek, and make preparation of packing list before beginning the trip.
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