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 then connects to Hyrule Castle. In the Twilight, it is infested with Shadow Insects and, in the Light World, with rats and Bulblins. The both times that the sewers are explored is as Wolf Link.

Overview
When Link initially gets captured by the Shadow Beasts, he is imprisoned in a jail cell inside the sewers. It is here that he meets Midna for the first time, who frees the young hero, and the two then proceed to make their way out of the sewers. As Midna and Link traverse the sewers, they must lower and rise 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 the game, after completing the Lakebed Temple, Link must once again go through the sewers to take the dying Midna to Princess Zelda. However, this time, he must access them via 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 overcome with Skulltulas as well as cobwebs that the young hero must burn in order to proceed. After passing this area, it will lead to the section that was explored before during the Twilight.

After Link reaches Princess Zelda in her chamber for a second time, the sewers will be inaccessible for the rest of the game.

Trivia
Castle Town Sewers are somewhat similar to the Secret Passage from A Link to the Past.