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Sacred spaces

Evidence suggests that among the Celts, "offerings to the gods were made throughout the landscape – both the natural and the domestic". There were also sacred spaces known by the Gallo-Brittonic word nemeton (plural nemeta), which typically meant a sacred grove or clearing. Greco-Roman accounts tell of the Celts worshiping at sacred groves, with Tacitus describing how his men cut down "groves sacred to savage rites". By their very nature, such groves would not survive in the archaeological record, and so we have no direct evidence for them today. Certain springs were also seen as sacred and used as places of worship in the Celtic world. Notable Gaulish examples include the sanctuary of Sequana at the source of the Seine in Burgundy and Chamalieres near to Clermont-Ferrand. At both of these sites, a large array of votive offerings have been uncovered, most of which are wooden carvings, although some of which are embossed metal.

During the Iron Age, the Celtic peoples of Gaul, Belgica and Britain built temples comprising square or circular timber buildings, usually set within a rectangular enclosure. Celtic peoples further east, in what is now southern Germany, Celtic peoples built rectangular ditched enclosures known as viereckschanzen; in some cases, these were sacred spaces where votive offerings were buried in deep shafts. In Ireland, religious buildings and enclosures were circular. According to Barry Cunliffe, "the monumentality of the Irish religious sites sets them apart from their British and continental European counterparts", the most notable examples being the Hill of Tara (Temair) and Navan Fort (Emain Macha).

In many cases, when the Roman Empire conquered Celtic lands, earlier Iron Age sacred sites were reused and Roman temples built on them. Romano-Celtic temples (Latin: fanum) are found only in the northwestern Celtic regions of the empire. They differ from classical Roman temples, and their layouts are believed to be hugely influenced by earlier Celtic wooden temples.