The dam which probably conforms best to the strict pattern of retaining wall is the one known as La Pared de los Moros (The Moors’ Wall)  .It is located near Muniesa (Teruel) in a secondary waterway, the Arroyo Farln, the rightward tributary of the river Aguasvivas  , which at the same time is a branch of the Ebro, also on the right bank (Arenillas, Daz-Guerra y Corts 1996).Built in the era of Augustus and rebuilt and repaired several times, this dam has a peculiar feature which makes it even more interesting: the preserved structure is an important reconstruction of a previous structure of completely different form. The first dam raised on the closure site of Almonacid must have been formed by three arches, one central and two side ones leaning against two large buttresses (Arenillas, Daz-Guerra y Corts 1996).This first dam must have been breached quite early, even perhaps in the later phases of construction and it also must have been rebuilt at once, its original structure being substantially modified, becoming the typical straight gravity dam.Most of the dams built in Spain by the Romans –and particularly the largest ones– can be gathered together in three main areas: the basin of the river Ebro, especially the right bank, whose focus may be located in Zaragoza (Caesaraugusta); the area of Mrida (Augusta Emerita) along the basin of the river Guadiana, and the left bank of the river Tajo in some points near Toledo (Toletum).The natural regulation of the rivers flowing in these sections of the Spanish mainland is low or very low, basically as a consequence of the unequal distribution of annual precipitation (Arenillas 2000); these climatic conditions forced the construction of reservoir dams.
The most important element of this system was the core of opus caementicium, whose purpose was to comply with the objective of retaining the water.
The definitive dam of Almonacid is a retaining wall, highly reinforced in its main part, with a thinner, short block on the left edge, where the weir is.
The main part of the structure –very robust- encloses the deepest area of the valley and consists in section of a rectangular central body and two stepped faces; downstream the stepping is double.
Other elements, though not always, were added to ensure or complete the stability of the system.
The Roman retaining wall is a very simple concept: a lime concrete core (opus caementicium), framed by two wall sections made of masonry (opus incaertum) or ashlar (opus quadratum).