The incorporation of the Catalan Counties to the Aragonese Kingdom meant that the new conquered territories would be linguistically heterogeneous. Catalan, in the Eastern region, Aragonese in the West. When the Aragonese reached the Southern region of Murcia, the Aragonese had no peoples to repopulate with. The territory was ceded to the Kingdom of Castille, and so, the Aragonese language started it's recession.
The spread of Castilian, now known as Spanish, as lingua franca in the Peninsula, together with the protective effect that Aragonese played for the Catalan language, meant that further recession would follow. The key moment in the history of Aragonese was when a king of Castilian origin was appointed.
The annexion of Aragon by Castille and suspension of all capacity of self-rule from the 16th century meant that Aragonese, while still widely spoken, would be limited to a rural and colloquial use, as the nobility would choose Spanish as a symbol of power. Aragonese reached its most dramatic point during the rule of dictator Franco in the 20th century. Pupils would be beaten in schools for using it and legislation forbid the teaching of any language that was not Spanish.
The Constitutional rights voted by the people in 1978 meant an increase in the literary works and studies conducted in and about the Aragonese language. However, it might be too late for it. Nowadays, Aragonese is still spoken natively within it's coremost area, in the Aragonese mountain ranges of the Pyrenees, and learnt as a second language by other inhabitants of the country in areas like Uesca, Zaragoza, Exea or Teruel. According to recent polls, all together they only account to around 30,000 speakers, making of this language one of the closest to extinction currently in Europe.