Your model could be Egypt - for millennia the Nile's regular floods brought brand new fertility and enough water to what was otherwise pretty barren surroundings. Your challenge might be why anybody would live in the NOT- fertile bits.

Or maybe the Niger River -- the adjoining Sahel region is pretty much man made or man-intensified near-desert, And the river brings life. Picture an area like that, where cities exist because of what ONCE was there. Your volcanos could provide both the MiracleGro and some of the barrenness. Say, where ash fall combined with other nutrients was good, vs . Where it choked out life or contributed to huge erosion or permitted overmuch electrification was bad. No lie - electrification. Ash is somewhat conductive. During/after the Mt St Helens ash fall utility workers had to be VERY careful not to take long steps while working around substations, else they could get very nasty shocks. Supposing some natural source of electricity, and some ash covered places would be "magically" uninhabitable...

You have some possibilities with the mountains causing rainfall, but as you note if they fully blocked rain, they'd also make travel difficult. Natural fortifications, as it were. How about if most of the continent was arid, and the fertile areas were due to springs and oases due to underground aquifers? The advantage could be partly technical - the secret of building pumps good enough to bring up agricultural amounts of water being jealously guarded. Maybe in many places an inferior kind of pump could bring up enough to barely support a population, but not enough to grow much.

Or let everywhere be lush and fertile, but only in a few places was some poison LACKING... Zucchini all around, but death to eat them. The Granary exports clean corn at ruinous prices to feed the rest of the continent's horses. Over in another area the huge wealth of mines permits expensive hydroponic growing. Another place has clean soil but little water, and has to build and defend canals and aqueducts to be able to grow food. Other miners have a fungi- based ecosystem, grown in played-out tunnels. A lake harbors edible fish - tough to protect from The Roving Fishbandits.