Simplify the following text for a Grade 7 native English-speaking person.
Original: Begun in AD 122 , during the rule of emperor Hadrian , it was the first of two fortifications built across Great Britain , the second being the Antonine Wall , lesser known of the two because its physical remains are less evident today .
Simplified: Begun in AD 122 , during the rule of the emperor Hadrian , it was the first of two fortifications built across Great Britain . The second was the Antonine Wall , the lesser known of the two .