Short answers, pulled from the story.
The Callaeci were youths who sang barbarian songs in their native tongue and stamped the ground rhythmically while playing sonorous shields called aegides. Strabo lists them as occupying a considerable part of the mountainous country along the northwestern Atlantic coast.
A battle at the river Douro took place in 137 BC between the Callaeci tribe and Roman forces where Paulus Orosius states that 60,000 strong Callaeci faced the Romans. Decimus Junius Brutus returned as a hero after winning this engagement which gave him his surname Callaicus.
The final conquest occurred under Emperor Augustus from 26 to 19 BC during the Cantabrian Wars when resistance included collective suicide rather than surrender among some groups. Mothers killed their children before committing suicide while crucified prisoners sang triumphant hymns.
Governing centers were Lucus Augusti at Lugo, Bracara Augusta at Braga, and Asturica Augusta at Astorga with administrative areas including Conventus Lucensis, Conventus Bracarensis, and Conventus Asturicensis respectively. Pliny records ASTVRIA ET CALLAECIA as a combined entity in written sources alongside Legatus iuridici inscriptions.
On the night of the 31st of December 406 AD several Germanic barbarian tribes swept over the Roman frontier on the Rhine where the Vandals Alans and Suebi advanced south pillaging Gaul before crossing the Pyrenees. The Suebi took part of Gallaecia where they later established a kingdom after the Vandals and Alans left for North Africa.