I forgot to include the following statements. I really hope you can have a look at them, too.

1)The Celts practised (?) agriculture.
2) The Romans improved the Celts' way of living (?) by bringing (with them/introducing) their civilisation, their language and a new legal system.
3) With the retreat (withdrawl) of the Roman troops the Romanized Celts had to fight against some Germanic tribes, namely the Angles, the Saxons and the Jutes.
3)The Anglo-Saxons were organized in family groups called clans, where the most important social bond was loyalty to the other members and to the lord of the tribe. (I need to rephrase this).
4)The Romans were succeeded (followed) by some Germanic tribes called Anglo-Saxons.The Roman invasion was followed by the Anglo-Saxon.
5)Thanks to Pope Gregory I The Great Christianity was brought back to England and monasteries progressed (I need a synonym: advanced?).
6) The most important characteristic of the Romans was its towns, connected by roads, many of which are still in existence to this day.
7)The most influential figures/members (not people) in Celtic society (also: among the Celts) were the Druids.
8) The Druids were the most important. The Anglo-Saxons were sent away (??) (were defeated?) by the Vikings.

1) The Celts practised agriculture.

2) The Romans improved the Celts' way of life by instituting their civilisation, their language, and their legal system.

3) With the retreat of the Roman troops, the Romanized Celts had to fight against some Germanic tribes, namely the Angles, the Saxons, and the Jutes.

3) The Anglo-Saxons were organized in family groups called clans, in which the most important social bond was loyalty to each other and to their chief.

4)The Romans were followed by some Germanic tribes called Anglo-Saxons.

5)Thanks to Pope Gregory I, Christianity was brought back to England and monasteries continued to be built and developed.

6) The most important contribution of the Romans were the towns and connecting roads they built, many of which are still in existence to this day.

7) The most influential figures among the Celts were the Druids.

8) The Druids were the most important.<~~The most important what? The most important among whom?

The Anglo-Saxons were defeated by the Vikings.