The Bank of Spain did not allow the production crew to visit the real building to replicate it on set. The interior sets were mostly fictional. Acording to the team they used "a lot of gray and a dictatiorial flavour" to build the sets, inspired by the "Valley of the Fallen", a Francoist regime monument.
When the Police scope out the entry points using optics, Suarez identifies the machine gun as a "Browning", when clearly it's an MG42. Later on in the series, the gun sounds like a 30 or 50 caliber weapon firing (a slow thud-thud-thud) when in reality the MG42 fired at 1200 rounds a minute, nearly double the amount of the Browning's 650 - 850 rounds a minute.
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