Castle's Waterway

Castle Town Sewers is a location in Twilight Princess. As its name suggests, it is the underground waterway of Hyrule Castle Town which connects to Hyrule Castle. In the Twilight, it is infested with Shadow Insects and, in the Light World with rats and Bulblins. Link only explores the sewers in his wolf form.

Features
When Link is captured by the Shadow Beasts, he is imprisoned in a jail cell inside the sewers. Here, Link meets Midna for the first time, who frees the young hero. Midna tells Link that she wishes to introduce him to someone, and the two begin searching for an exit from the sewers. As Midna and Link traverse the sewers, they must lower and raise the water levels of the underground passageway in order to eventually exit through a small tunnel at the far end. At the edge of the sewers is a long, spiraling staircase which Link must cross via ropes to reach the rooftops of the castle. By jumping from rooftop to rooftop, Link eventually reaches the tower in which Princess Zelda is imprisoned.

Later in Link's quest, Midna is gravely injured by Zant, and Link must once again go through the sewers to take the dying Midna to Princess Zelda. He enters them through a secret passageway from Telma's Bar, which leads to Jovani's House and then to the underground waterway. Here, Link explores a new section of the sewers which is infested with Skulltulas as well as cobwebs that the young hero must burn in order to proceed. This connects to the area in which Link was imprisoned near the beginning of his quest.

Once Link reaches Princess Zelda for the second time, he will be unable to return to the sewers again.

Trivia

 * Castle Town Sewers are somewhat similar to the Secret Passage from A Link to the Past.
 * It is believed by Team Beta Triforce that, in Ocarina of Time, the sewers were the original way to sneak out of Hyrule Castle, as in A Link to the Past.