Complex sentence: The Lowlands had been Brythonic Celtic, with the southeast becoming Anglian, and Galloway and the western seaboard becoming Norse-Gaelic, then by 1034 the Kingdom of Alba had expanded to bring all but the last area under Gaelic Celtic rule.
Split into simpler sentences: The Lowlands had been Brythonic Celtic, with the southeast becoming Anglian, and Galloway and the western seaboard becoming Norse-Gaelic. Then by 1034, the Kingdom of Alba had expanded to bring all but the last area under Gaelic Celtic rule.