Simplify the following text for a Grade 8 native English-speaking person.
Original: The first walls were wood and earth palisades built to defend the Roman fort in 79 , although these were replaced by stronger stone walls between the 1st and 3rd century .
Simplified: The first walls were built to help the Roman fort in 79 , although these were replaced by stronger stone walls between the 1st and 3rd century .