At the height of the Celtic empire in 4th Century BC, they sacked Rome and many of her Baltic states. Even though the Celts tried to hold their new Roman territories, their lack of unity was their downfall, and Rome and her allies finally managed to defeat the great Celtic armies. By 1st Century BC Julius Caesar and his troops had managed to kick the Celts out of most parts of Europe. And by 1st Century AD, all of the Gauls, what today are France, Spain and Germany had been taken by the Rome. The only independent Celtic estates by this time where the ones in what was then called Brittani, or today's England. .
The Celts began to appear in England at around 2nd Century BC, but their full "invasion" of people did not arrive until about 5th Century AD. As before mentioned, the Roman counter attack had left thousands of Celtic tribes without a real home and Brittani, the only standing Celtic territory seemed to be the wisest option for adopting as a new home. This time was crucial for the adoption of certain Celtic words and sentence structure between the two peoples. This state of peace and cultural interchange did not last long.
This new safe haven did not last long either, as the Romans pushed into England, terrorizing the Celts, and making them flee to corners of the island.
There is very little known of written works in the time between the Anglo-Saxon invasions and the first Old English manuscripts. This makes Celtic lore so important in knowing what exactly happened at this time. The only written piece in "Old English" that is still around today is the poem "B eowulf".(Picture of Poem/ Explanation).
Luckily, the Celts outlived the Roman conquest by staying in remote areas of the island and by repelling Roman attacks. When the Romans finally left England, a fine line of belonging had been drawn between the Britons the natives of Brittani and the Celtic, the peoples who took the island as their home.