Complex sentence: Britannia Inferior extended as far north as Hadrian's Wall which was the northernmost border of the Roman Empire--although the Romans invaded modern-day Northumberland and Scotland beyond it, they never succeeded in conquering the reaches of Britain beyond the Tyne.
Split into simpler sentences: Britannia Inferior extended as far north as Hadrian's Wall, which was the northernmost border of the Roman Empire. Although the Romans invaded modern-day Northumberland and part of Scotland beyond it, they never succeeded in conquering the reaches of Britain beyond the Tyne.