Allt y Gaer (Welsh for 'hill of the fort') is a Site of Special Scientific Interest (SSSI) in Llangathen community, Carmarthenshire, Wales.

It is a 3.5ha conifer woodland on the northern slopes of the Tywi Valley, which has SSSI protected designation because it holds the largest heronry in West Wales.

The SSSI designation aims to encourage as little disturbance as possible, especially during the February–June breeding season, whilst maintaining an abundance of nesting sites and food supplies.

Although the dense shade and extensive litter layer of needles and branches have completely suppressed the pre-plantation flora, the mature stand of trees has allowed a significant growth of various species of fungi; in particular the wood provides a habitat for Spotted Toughshank (Rhodocollybia maculata), Plums and Custard (Tricholomopsis rutilans) and Clouded Agaric (Clitocybe nebularis).

[3] Higher up the slopes, 350 metres (1,150 ft) to the north, is an Iron Age hillfort called Grongaer[4] which gives the name to the hillside, the woodland, and a nearby farm.