Roedd rhai o drigolion cyntaf Efail Isaf yn byw yn y Bryn Gaer uwchben y pentref ger Coed Hendresguthan. Mae'r bryngaer mewn safle amlwg gyda golygfeydd i bob cyfeiriad. Tybier iddo gael ei adeiladu tua 2500 o flynyddoedd yn ôl yn yr Oes Cerrig.
Dyma ddisgrifiad Comisiwm Henebion Cymru o safle NPRN 409855:
"A univallate oval hillfort crowns the west-facing summit of a projecting spur at 165m above sea level. The most obvious surviving rampart blocks the easiest approach from the east, lying directly across the neck of the promontory. On the remaining sides the hillfort appears to be enclosed by scarp banks. The fort measures approx. 191m east-west by 94m north-south enclosing some 1.42 hectares. The site was first recognised from the air during RCAHMW aerial reconnaissance in the dry summer of 2006 (AP_2006_3207) when parching picked out the main eastern rampart against the hill pasture."
(http://www.coflein.gov.uk/cy/safle/409855)
Efail Isaf o'r bryngaer
