Liberating the Portage

After dealing with the goblin den, Karl leads you uphill a few hundred yards to a narrow defile running along the side of the mountain. The trail he leads you on runs fairly evenly above the river far below, partly underground, partly through deep ravines, and partly across open hillside. At one of the open spots, he pauses to point down at the waterfall below, with the normal portage route visible on the far side of the river. From the far side of the stream and a higher elevation, you have a great view of the portage route as a whole. From the landing below the waterfall, there is a broad path cut into the rock rising in a series of steps to a ledge that runs around the cut of the falls. In places it has been built up to allow passage from one ledge to another, and other parts have been chiseled out to allow easier passage.
With Karl's warnings in your mind, you quickly locate a waft of smoke rising from a small ravine above the waterfall and the portage. You notice it is located above and behind a serious outcropping of rock that protects it from view from the river or the portage. Your position higher up the mountainside and across the river gives you a chance to study the bandits' camp and ambush position. The camp is well-concealed, built from carefully positioned stones covering a brickwork building beneath. From ground level, it would be almost impossible to spot.
Karl points out that the path you are on continues another mile upriver before returning to the water below. There is a lone sentry lying atop a large boulder looking down on the waterfall and the landing below. They don't appear to be paying any attention to traffic coming from upriver. There are no other people visible. Karl suggests that you might be able to continue on to the river, then cross and come at them unaware. He believes there is an old goat trail on the far side of the river that can take you to a small ledge he points out 30 feet above the bandit camp site.
It is possible to skirt the camp and approach it from upriver, avoiding the lookout and taking the building by surprise from the ledge above it. The party would have to make survival checks to locate the goat path, athletics checks to climb its steep slope, and stealth checks to avoid alerting the bandits in the hideaway. With dusk approaching quickly, they could get into position and wait for an opportune moment after dark to attack in order to surprise the bandits in their sleep. They can roll stealth checks with advantage against the passive perception of the bandits. (Tomorrow I'll need to make up a bandit gang, and get some stats for Karl.)