The wardrobe in line 46 works on two levels. First, it's a reference to the huge piece of furniture in C.S. Lewis's The Lion, the Witch, and the Wardrobe that became a portal to the world of Narnia (where Aslan the Lion and the White Witch lived). By association, the wardrobe is shorthand for a place of passage into enchantment.
In the speaker's case, this land is a world devoid of scary things (unlike Lewis's Narnia). It's a place of promise and better times (i.e., Spring). So what is the wardrobe in this poem? We might identify it as an actual thing—a book, the doors to the library, the stereoscope in line 25—but it could be something less tangible.
We know that the speaker loves the world of the past in Knoxville with her library and her grandmother. No matter what might have been going on in the world around her, she felt safe, loved, and intellectually stimulated. It obviously had a strong effect on her future life. It's just possible that the "wardrobe" was the opportunity, the springboard to Good Things that the love and care of Mrs. Long and her grandmother gave her.