Bastion of Thunder - Tower Access Quests
A majority of Torden, The Bastion of Thunder is available to you without having to complete any keys or quests. You'll be able to access the Ice, Fire, Earth and Air wings as well as kill the bosses at the end once you're sufficient level to enter the zone. In addition you'll be able to access the Stormrider's territory which you reach by taking a teleport pad at the end of any of the 4 wings.
In order to explore more of the Bastion of Thunder you'll be required to complete one (or two, depending how deep you want to go) quests. The first quest you must complete is for access to each of the four different Towers (you can find the clickies to reach these towers in the Stormrider's Area). The second quest you must complete is to reach Agnarr's tower, the raid boss of this zone.
The first of these quests, which allows you access to the Bastion of Thunder Towers is about as straight forward as quests come. What you need to do is kill one of the named bosses at the end of one of the four different wings and loot a Ring of Torden. It's a container that has 4 empty slots and a Combine button.
Now what you'll want to do is kill enemies in all four wings until you find 4 different types of gems. You'll need a Blizzard Gem, Tornado Gem, Sandstorm Gem and Lightning Gem. Once you've obtained one of each you'll place all four of them into the Ring of Torden and hit combine.
After you combine all four gems in the Ring of Torden you'll receive the Enchanted Ring of Torden. With this in hand you'll want to venture down to the Stormrider's area and over to one of the clickies that take you up to the BoT Towers. Once you click on one of the objects to be transported into the towers the Enchanted Ring will be added to your Keyring.
Note: Once your progress far enough to no longer need flags to the BoT Towers - the Enchanted Ring of Torden will not be added to your keyring upon successfully clicking up to a tower.
Access to Agnarr's Tower
With the Tower Access quest out of the way the next quest for you to do is for access to Agnarr's tower. Agnarr is the raid boss of this zone and there's no real reason for you to complete this quest unless you plan to fight him at some point. This quest follows almost the exact same formula as the previous one. For this quest all you need to do is...
Travel into one of the four towers; kill one of the named enemies at the end of the tower just before the teleport pad that takes you back to the Stormriders. He'll drop an Unadorned Symbol of Torden which is a 10 slot container with a combine button.
Last time you had to collect 4 different gems to combine into these container, one from each wing. This time you need to collect 4 different Spheres to combine into this container - one from each wing (but they drop much more commonly from the towers). Lightning Sphere, Blizzard Sphere, Tornado Sphere and Sandstorm Sphere.
With all four elements combined you must form Captain Planet. To do so you'll need to place all four spheres into the Unadorned Symbol of Torden. Hit combine and you'll be greeted with our Hero, Captain Planet. I jest, but once you do hit combine you'll receive a small Symbol of Torden coin as a reward.
This coin grants you access to Agnarr's Tower which is the final location of Bastion of Thunder. In order to add this key to your keyring as well as gain access to Agnarr's Tower you'll want to click on the object that is found in the center of the Stormrider's area surrounded by four stone gargoyles which don't attack unless attacked first.