Short answers, pulled from the story.
Ancient sources assigned multiple meanings to the Latin noun limes, including a path marking field boundaries or a channel like a stream. After the third century, this term became an administrative label for a military district commanded by a specific official.
Hadrian's Wall construction began in 122 AD within Britannia. A garrison of 50,000 soldiers defended this massive stone barrier against Caledonian attacks.
The province of Dacia sat completely on the far side of the Danube river while Germania Magna existed as land between the Rhine, Danube, and Elbe rivers. Continental European borders generally followed major rivers like the Rhine and Danube.
The Sasanians succeeded the Parthians between 224 and 226 AD. This new dynasty became one of four leading world powers alongside Rome for over 400 years.
Qasr Ibrim marked the southernmost point near modern borders between Egypt and Sudan. Roman control extended many kilometres into Africa up to Syena and Berenice.