Route 11 links Route 12 to Vermilion City. There are only a few Trainers here, but it does lead back to the Snorlax just outside the city. To wake it up, tune the Pokégear's radio to the Poké Flute Channel and examine snoozing creature to place the radio near it. Annoyed, it attacks the nearest living thing. After the battle, Diglett's Cave is no longer blocked, leaving you free to continue through the tunnel.
Route 2 connects Viridian City to the south and Pewter City to the north. Visit the man in the house to get a Nugget, then pass through the gate and cut through the trees to reach the west side. Hike through the remains of Viridian Forest on the way northward.