What is the meaning of Agger?

A double tide, particularly a high tide in which the water rises to a given level, recedes, and then rises again (or only the second of these high waters), but sometimes equally a low tide in which the water recedes to a given level, rises, and then recedes again

Synonym of earthwork in ancient Roman contexts, particularly a defensive wall or mound.

to agree

earthwork, particularly defensive ramparts or bulwarks, dykes, dams, causeways, and piers

rubble or earth used or intended for such earthwork

any rampart or defensive wall

Source: wiktionary.org