Is a plateau higher or lower than a coastal plain?

The answer is - The plateau is higher than the coastal plain.

The plateaus are highlands, on occasions higher than some mountains. They are predominantly flat or wavy, so they look like big plains on higher altitude. 
The coastal plains on the other side, are very low, close in height as the sea level itself, which actually makes them the second lowest plains after the depressions, so they are much lower than the average plateau. 

The investiture controversy began as a power struggle between pope gregory vii (1072–85) and henry iv, holy roman emperor (1056–1106).[2] a brief but significant struggle over investiture also occurred between henry i of england and pope paschal ii in the years 1103 to 1107, and the issue played a minor role in the struggles between church and state in france, as well.by undercutting the imperial power established by the salian emperors, the controversy led to nearly 50 years of civil war in germany, and the triumph of the great dukes and abbots. imperial power was finally re-established under the hohenstaufen dynasty. historian norman cantor: