Primera Guerra Civil Castellana
The wars resulted partly from the destabilising effect of the Castilian Civil War and the ensuing political weakness of the House of Trastámara. That the Seniory of Biscay effectively ceased to exist after 1370 and was subsumed in 1379 into Castile only exacerbated the effects of political anarchy on Biscay. The rise of the towns, notably Bilbao and Bermeo, caused jockeying for municipal power between the rich urban families. The landed nobility, enfeoffed in the heavily encastellated countryside, nursed feuds that dated back centuries. The Legizamon and Zamudiano had been enemies since 1270. Many minor noble families were caught in the feuds of the great families by ties of marriage.