This chapter examines military activity and violence, as manifest in aspects of the archaeological and historical records for medieval times in the central Sahel and north-west Europe. It explores the contribution of military innovations to political centralisation, the prevalence of fortifications and town walling, and the widespread occurrence of slave raiding. It highlights the individual agency and motives in attracting followers for one's army, in building up stables of horses for prestige and power, and in building walls for purposes both definitional and defensive.