Vale nods vigorously. "Agreed. The walls will be manned by their better fighters, but if I know the Kreegs, the ones off-duty will be easier to take down. When they take their eye off the ball, they get sloppy."
The caves behind the waterfall are an obvious way to sneak inside.
Jakardros also points out the castle's sluice gate, on the south wall, not too far from the waterfall. "The pipe yonder should be big enough, barely, to get through the wall and into the castle. Not a very pleasant trip, as you can imagine. It's the drainage for the keep's sewage and refuse."
Nich, you notice behind the outer curtain wall that you see occasional ogres coming and going from a large wooden building. The rangers point out that the structure is the keep's New Barracks, but they abandoned it due to the danger from fire. Anyone sleeping inside would likely burn to death very quickly.
Baiting and luring out a company of ogres could prove useful. You could also do some combination of these things, maybe even splitting up the party to set up a distraction.