Arguably the most referenced chapter. The GURPS Low-Tech PDF completely revises the armor rules from the Basic Set.
This section provides rules for the harsh realities of pre-modern life: gurps low-tech pdf
From castles to cottages. This chapter details construction materials, siege engines (trebuchets, battering rams, siege towers), and fortification DR. It also includes the "Maintenance" rules—how long it takes to sharpen a sword or repair a leather jerkin. Arguably the most referenced chapter