Candy Tunnel is one of the easiest Obstacles in battle run, it's appears as a small tunnel with candy on it.
This obstacle only appears in the Candy land stage.
This obstacle is mostly seen in the middle of a set of candy spikes or other obstacles, they make it hard sometimes but they are really easy to avoid.
You can't go in the Candy Tunnel because it's to small of a tunnel and if it would kind of be like the open gate in the Castle Stage.
The candy tunnel appears to have chocolate, mint, and other candy related items on it.
If you want more info, maybe you would like to look at the Candy Land(Stage) article.