Theory:

The happening of the same letter or sound at the beginning of adjacent or closely related words.
 
Alliteration happens when words that start with the same sound (not just the same letter) are used repeatedly in a line or sentence or even a phrase. The sound is a consonant, and the words don't have to be next to one another.
Example:
1. Shut the shutters before the banging sound makes you shudder.
2. Go and gather the green leaves on the grass.
3. Please put away your paints and practice the piano.
The alliterated words in the poem "The Poem of Adventure" are:
 
1. that even through the ages are held in high esteem.
2. in a one to one combat defying death.
3. a new step you bounce a bit.
4. highlighting aspects of human wisdom related to virtue and vice.
5. because those that live their lives in adventure see the world in an extraordinary special way.
6. It was mainly for show, love and glory they deemed themselves being worthy to capture the heart.
7. And in fighting for their country, faith and king noble impressions on people’s minds.
8. the lessons to be learnt are those of goodness and sacrifice.
9. as it often happens a situation comes when we’re asked to do something.
10. every time you take a new step you bounce a bit.