Although there is some evidence of earlier inhabitation in the region of Utrecht, dating back to the Stone Age (app. 2200 BCE) and settling in the Bronze Age (app. 1800–800 BCE), the founding date of the city is usually related to the construction of a Roman fortification (castellum), probably built in around 50 CE. A series of such fortresses was built after the Roman emperor Claudius decided the empire should not expand north. To consolidate the border the limes Germanicus defense line was constructed  along the main branch of the river Rhine, which at that time flowed through a more northern bed compared to today (what is now the Kromme Rijn). These fortresses were designed to house a cohort of about 500 Roman soldiers. Near the fort settlements would grow housing artisans, traders and soldiers' wives and children.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What was first built in 50 BCE
unanswerable