The line "I came across a fallen tree" refers to a specific fallen pine in those woods where the three band members were famously photographed together around the age of 11.
They represent the "simple things" mentioned in the lyrics—innocence and pure moments that are often lost as one grows older. Keane - Somewhere Only We Know (Official Music Video)
These creatures are known as "Roxleys" (a play on Tim Rice-Oxley's name) or "Simple Things". The line "I came across a fallen tree"
The "pathway" mentioned in the lyrics is the actual trail between Tim Rice-Oxley's parents' house and the local pub. Video Themes and "The Simple Things" Keane - Somewhere Only We Know (Official Music Video)
The "somewhere" referenced in the song is a real location in the band's hometown of .