I was thinking to place the Keep on the Borderland in the Sheldomar Valley, at the limit of the Principality of Ulek and have the Cave of Chaos in Drachensgrab mountain in the Pomarj, where orcs, goblinoids and other monsters can be found in mass.
Very nice. If I recall correctly from the Gazetteer map, it's near the town of Threshold. I'm actually transposing Threshold into my Shield Lands earldom as the nearest larger settlement. And while the keep will be near a major trade route to another earldom (I need to look at Karen Meyer's maps again but I think it's referred to as the Old Aerdy Road maybe?), I do plan to incorporate it being a bit on a divergent path so the road to the keep has some patrol waystations (idea borrowed from Hackmaster's Frandor's Keep module which I believe is a serious rewrite of their Little Keep on the Borderlands module) placed along the route.I'm setting mine in the Grand Duchy of Karameikos, in the foothills of the Altan Tepes Mountains, in a place labelled on the Gazetteer map as 'Castellan Keep'. Mystara is back baby!