The Medieval collection is a package of ten Bouche shield icon themes of different colors. The Bouche icon themes are built on top the Gnome and Adwaita icon themes.
To install, extract the Medieval directory and move the individual Bouche subdirectories to ~/.icons/ (or /usr/share/icons/).
*** Do not skip the Bouche-Black directory. This is the base of the installation ***
Then go to Appearance and select the new icon theme of your choice.