Winx Club Forever
As stated, the Tree is the keeper of all doors. This is where the Key to the Black Gate is stored since being sealed ages ago. Upon entering the tree it is pitch dark but the keys illuminate the inside. The keys floating inside in a neat fashion. However, whatever forces that keep the keys afloat can be easily disrupted and all the keys fall to the bottom.
The Tree of Portals is extremely tall and is a pale tan color. Its base is very wide and thick, has large roots and is covered with green moss and fungi. A single branch winds around the tree, with mushroom tops as stairs that lead into a keyhole, which is an entrance into the Tree itself, of which are many. From the base up, it gets skinnier and skinnier.
The Secret of the Lost Kingdom
The Winx (except for Bloom, because her Enchantix is incomplete) enter the tree in their Mini Winx form, along with Lockette, Amore, Tune, Digit, Chatta and Piff. They are trying to find the key to the Black Gate, the Portal of Obsidian. Lockette finds it with her scepter that led her to it.