Between 250 and 225 BC, during the Iron Age, the Parisii, a sub-tribe of the Celtic Senones, settled on the Île de la Cité and on the banks of the Seine. At the beginning of the 2nd century BC, they built an oppidum, a walled fort, either on the Île de la Cité or nearby (no trace of it has ever been found), and they built the first bridges over the Seine.[2] The settlement was called "Lucotocia" (according to the ancient Greek geographer Strabo) or "Leucotecia" (according to Roman geographer Ptolemy), and may have taken its name from the Celtic word lugo or luco, for a marsh or swamp.[10] It was the easiest place to cross the Seine, and it had a strategic position on the main trade route, via the Seine and Rhône rivers, between Britain and to the Roman colony of Provence and the Mediterranean Sea.[11][12] The location and the fees for crossing the bridge and passing along the river made the new town prosperous,[13] so much so that it was able to mint its own gold coins, which were used for trade across Europe. Coins from the towns along the Rhine and Danube and even from Cádiz in Spain were found in the excavations of the ancient city.[14]