It is dubbed “Guantanamo” by the 6,700 employees who work here, perhaps because of its relative isolation, perhaps because of the security measures that visitors must pass through to get in, but that is where any resemblance between the notorious US maximum security prison in Cuba and Europe's biggest financial campus ends. Welcome to Santander City, home to the eurozone's biggest bank.

In a world where size matters, the sheer scale of Banco Santander's head office speaks volumes about the bank and its ambitions. Built on a 150-hectare site, near Boadilla del Monte, a 45-minute drive northwest from Madrid, this is the physical representation of a modern, 21st-century bank.

Inspired by Wachovia's American corporate campus near Charlotte, North Carolina, Emilio Botín, Santander's chairman, realised that this was a way to transform the business's image. “Santander Group City is more than just a headquarters; it shows that as a group we are a leader, modern and international,” he said.

The idea crystallised long before Santander went on a shopping spree in which it snapped up Abbey, Bradford & Bingley and Alliance & Leicester in Britain, and long before the credit crunch that has so stymied international banking, a financial squeeze that this month prompted Santander to launch a €7.2 billion (£6.14 billion) rights issue to strengthen its core capital.

Before making the move in 2004, Santander had been hampered by its charming but antiquated headquarters in central Madrid. “Yes, I do miss the shops,” one employee said of the bank's new detachment, and the bank's employees had laboured in 23 different sites in the Spanish capital, a hangover from the pre-merger era, before Santander had absorbed three of its competitors.

The old head office was an ornate Belle Epoque building, where time had stood still, liveried doormen lurked in attendance and Old Masters hung in the boardroom. It reeked of “old Spain”, when aristocratic families ran the country's financial institutions. But getting around was a nightmare, with employers often held up by Madrid's notorious traffic jams as they tried to get to meetings.

Today the first sight that greets visitors to the City, designed by Kevin Roche, the Irish-American architect, is a large chimney bearing the bank's white-on-red flame logo, making the grey granite giant appear like a factory, pumping out money. There is a supermarket, a designer hairdresser, a fully equipped sports centre with an Olympic-sized pool, tennis courts and a gym. Of course, this being the head office of a bank, an 18-hole golf course is essential to seal deals or merely to enjoy a few rounds. Not forgetting the five restaurants serving a variety of dishes, including Italian and a traditional buffet, and an executive restaurant for the bosses, a training centre and an on-site hotel.

The culture has changed, too. Gone are the siestas, the three-hour break between 2pm and 5pm. Lunch hours are down to one hour, but the working day, which used to stretch into the evening, has been reduced and flexible working is encouraged.

Gone, too, are the days when such luxury would have been enough for Santander. Keen not to miss a money-making opportunity, it agreed a €1.9 billion sale-and-lease back agreement in September, making the bank a tidy €600 million profit.

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