Pokémon Red and Blue/Viridian Forest

'The nature preserve is smack-dab in the middle of Route #2, the route that runs between Viridian City and Pewter City. Pokémon are plentiful within its borders, and many trainers come here to capture new specimens for their collections or to challenge other trainers. It's easy to lose yourself in Viridian Forest if you don't know where you're going.'

Through the woods
Viridian Forest serves as your first "maze" stage. Its hard to get lost in there, but first timers may wish to explore it slowly to get a feel for the layout. As you cross Viridian Forest, you'll be attacked by both wild Pokémon and various trainers. While encounters with wild Pokémon are random, trainers will only come at you when you cross their line of vision, so you have a degree of control over when they challenge you. Unfortunately, they are usually placed in positions that you have no choice but to pass in front of, but a few can be avoided by walking around them.

If you are having trouble defeating trainers in a given area, it probably means that your Pokémon aren't strong enough and need to be leveled up higher. The highest level Pokémon that a trainer in Viridian Forest has is level 9 (level 10 in Yellow), so as long as most of your Pokémon are level 8 or above, you should be in good shape.

The pets and the wildlife that you encounter in the forest may seem tough, but much of it won't be. Wild and challenger-trained and  can't attack, they can only harden (which can make for some long battle that wear down your Pokémon's PP, so be careful). Never try to capture a wild Metapod or Kakuna. Instead, capture and  and level them up so they evolve into Kakunas and Metapods that retain their offensive abilities. They evolve fast: Both will be at their third stage of evolution by Level 10!

s are extremely rare in Red and Blue (and non-existent in Yellow, except for your own) and will take extensive periods of walking around through grass to find them. It isn't necessary to capture one, but they can deal with low-end Flying types and Water types.