Tunnel to the Tower

The Tunnel to the Tower is a location in Spirit Tracks. As its name implies, the tunnel is a secret passageway situated behind Hyrule Castle that leads to the Tower of Spirits. In order for Link to access the way to the tunnel, he must obtain the Recruit's Sword from Guard Captain Russell from the castle training room. Once he obtains it, he can talk to the Castle Guard standing behind Hyrule Castle to let him through. The entrance to the tunnel, however, has been blocked off due to a recent rockfall, forcing Link to bomb the nearby wall to reveal the tunnel's entrance.

The inside of the tunnel is a sort of mini-dungeon consisting of three floors infested with Keese, Red ChuChus, and Spinuts. Link must solve various puzzles to get to the exit of the tunnel and thus make his way to the inside of the Tower of the Spirits, where Zelda and Link meet Anjean and learn the true reason of why the Spirit Tracks have been disappearing.

If Link returns here after obtaining the Spirit Train, the roof at the end of the tunnel will have collapsed, making it impossible to access the Tower of Spirits from there again.

Trivia
Interestingly enough, even though it is in the tradition of the Zelda franchise to equip Link with a sword and a shield before he is allowed to venture out on his own. Link is not required to have a shield to pass into the tunnel, let alone complete the entire game. This is also the case for Oracle of Seasons, where it is required to obtain a sword, but not purchase a shield, as well as Oracle of Ages, where Link is given a sword, but not required to purchase a shield, except in a Linked Game, where he is given a shield and already has a sword.