Leave León on 7ma Calle SE (just check the map!) where just beyond the city outskirts the tarmac ends. This is a smooth, dirt, sometimes-pebbly-sometimes-sandy road that offers tantalizing views of Volcán Momotombo straight ahead in the distance. Pass through Las Mercedes village after about 18 km and then continue another continue another 19 km to NIC-22 . Go north for about 2.5 km and then merge onto NIC-44B for the final 3 km to the ruins of León Viejo.
This is where I ended but if you’re adventurous, consider continuing north a kilometer or so and then taking Hwy 236 east along the base of beautiful Volcán Momotombo and along the shore of Lago Xolotlán. Tours to hike the volcano use a 4-wheel drive and my guess is that the road really starts to deteriorate after the Momotombo Power Station. If you tackle this route, I’d love to hear how it was.