The piece is in the key of E minor and consists exclusively of the basic harmonies E minor, A minor and B7. A special feature is the melodic lead of the bass line in the rhythmic scheme of the milonga. You can see that the lowest notes of the picking pattern are consistently played on the first, third and seventh note of the bar. This corresponds to the basic 3-3-2 rhythm of the Milonga as described above.
The second part of the song is technically more demanding: the melody goes beyond the first position. The incorporation of the (very guitar-typical) interval of a sixth and triads in the high register creates a change of mood before the final part returns to the first position.