Pokémon Red and Blue/Victory Road
Enter the league's front door to find yourself in the rocky cavern known as Victory Road. The trick here is to push the loose boulders onto the round switches to open up new paths. You must open the path to ladder D on the second floor. This leads to ladder E, which leads to the exit.
 The road to success
Victory Road is a lot like Seafoam Island. Once again, you'll need a Pokémon with Strength, and you'll need to do a lot of pushing rocks around. This time, there's a twist: The rocks must be pushed onto eye-shaped switches, which then makes those weird little red and white blocks disappear.
The first few are easy, if you're careful not to get the boulders stuck against a wall. But when you get down to level 3, you'll need to push a block all the way from one side of the dungeon to the other. Then loop around to the bottom, and push the next block down the hole to level 2, to open the final switch.
You'll need a strong and versatile team to handle the many Junior Trainers here, but go heavy on Water Pokémon for the random encounters and Moltres.
You can also find a hidden Ultra Ball in the lone rock near Moltres's location, and a Full Restore just before the exit, immediately to the right of the final ladder, in a rock.
Moltres, accessible though the C ladder on F3, is the last of the Ancient Legendary Birds. But fortunately, he's the easiest to catch. If you've been doing the Birds in order, you should have an Articuno now, and one Ice Beam from him should put Moltres directly into the red zone. Now you can toss Ultra Balls until you get a hit (if you're having trouble, reset and keep trying until Ice Beam freezes Moltres, making him a cinch to catch, or try using Hypnosis to put him to sleep). Note, however, that whatever you do, do NOT…EVER… use Blizzard. Blizzard can and will knock out Moltres in one shot, especially if you try using it from an Articuno, and if you're trying to catch this legendary bird (which you should DEFINITELY do), any and all work you used getting to that point will have been all wasted in one fell swoop. Be very careful that the attack selector is on the right attack.