★★★★★ This pinpointed location on Google Maps is by the Mont Royal parking lot. If you're here for the observation deck to enjoy panoramic views of downtown Montreal AND are arriving by car, definitely drive to this location. The observation deck is another 800 meters from here, and can only be reached by foot. Because this location sits at a higher elevation, the hike shouldn't be too daunting. My 55-year old aunt did just fine.
Of course, there are other ways to reach the observation deck, but all of them require some hiking. If you're downtown, you can start hiking at the foothills along Pine Ave or Cedar Ave. That hike is more uphill and might be daunting for some.
Wear comfortable walking shoes, bring water and provisions, and enjoy the view! There are visitor amenities at the observation deck, including restrooms, a gift shop, and a small cafe.