"Key of the Kingdom" by two-time American Poet laureate Ed Reed is full of wonderful imagery. Most of it is centered around the speaker's memories of being a child, full of imagination and ready to take on the world. 

One effective use of imagery is the picture of a medieval fantasy world created with everyday things but full of many other-worldly elements. Though the castle was only made out of big cardboard boxes, shields were just garbage can lids, and swords were made of simple lattice garden fencing, the imaginary enemies and adventures seemed vivid and real.

A world inhabited by goblins, ghosts and ghouls,

Dragons, trolls, witches, sorcerers,

Knights, fair damsels, wicked kings

And green - skinned, three eyed flops.

A world of enchanted geography -

Magic Forests,

Glass mountains

And fountains of youth. 

The author's use of details to create an imaginary world are one way to achieve effective imagery. We can picture any kids in any time creating just this kind of castle and fighting just these kinds of imaginary beings. There is joy and creativity, imagination and adventure in this image.

Two other effective images come at the end of the poem, but they are anything but joyful. The entire premise of the poem is the metaphor of a key, in this case a key to the kingdom of imagination, joy, and hopefulness. We all had it once, says the speaker, like when we, too, made medieval castles out of discarded television boxed and crossed little ponds as if we were Christopher Columbus sailing to a new world. Over time, however, our imagination and joy wither. 

The image Reed uses to portray this deterioration and loss is this:

We have lost the key

And it has perished with the rust of misuse

And neglect. 

This image of a neglected, rusty key is an apt visualization of that loss of active, jouful creativity and imagination. It has been left to decay in the realities of life and living. 

The final image in the poem is even more pointed:

Age is the grave yard

Of all our youthful hopes.

Dreams

And experiences. 

A cemetery is a place of sadness and loss, so visualizing old age as the place where dreams, hopes, joys, and experiences dies is a sobering but fitting depiction. In this image we experience mourning for all the childhood joys, hopes, and dreams that we who have gotten old have lost. Though it is a feeling common to all men, we are nevertheless saddened at the loss.

The final two images are strong, but their impact is intensified by the joyfulness and delight throughout the rest of the poem, as in the first image above. This poem as a whole is particularly effective because of that contrast.