Op. 111 "Lugubrious march" / "Marcha lúgubre" - S. Vega by S. Vega 

Try the interactive tutorial, or download the Sheet music here: musescore.com/...
Hello!
This piece is one of the saddest for me.
It reminds me of a mortuary burial.
It has a simple structure and harmonies.
The structure is A-B-C-A.
The A parts are the march itself. While the B part has a simple and powerful melodic development in octaves and seventh chords, the C part is somewhat more lyrical, and returns in a quiet way towards the coda.
In terms of harmony, it is always kept in the key of Bb minor, F minor or its nearby keys.
I hope you enjoy it. Regards!