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The knives were out for Francisco Mangado. The Spanish architect was hauled before the courts by the political wing of the Basque separatist group ETA. Lawyers from Herri Batasuna were charging Mangado with flouting national preservation laws at the site of his Baluarte concert hall in the northern city of Pamplona. Not that Mangado was alarmed by the threat of the two-year jail sentence demanded by ETA: he was more worried about meeting the unforgiving schedule he'd been set to build Spain's largest concert hall.

Mangado, 43, is part of a new wave of Spanish architectural talent. In 1998 he beat 150 architects, including many of Spain's most established practices, to win the competition to design the Baluarte. Until then, the largest projects he had undertaken were swimming pools and health centres. Suddenly, he was designing a 63,000 m² conference and exhibition complex in the city centre with a budget of €80m (£56m).

"When you have to face a project with so many political, social and economic problems involved, at first you are quite frightened," says Mangado. "You're not sure you're going to have the right answers."

The political problem threatening to land Mangado in jail had its origins in the proximity of the concert hall to one of Pamplona's architectural treasures – a 16th-century citadel built by Philip II. Meanwhile, its key city location demanded an astute organisational layout. Mangado's design was the jury's favourite because of the sensitive way it negotiated these two factors.

"Most entries in the competition were concerned with how to treat the building as an architectural object," says Mangado. "They were very autonomous, much more concerned with stylistic questions and less interested in how to put the building in the urban situation. Our project renounced the idea of spectacular architecture."

Having won the contest, however, Mangado found he had less time to build it than he had anticipated. "The schedule was very demanding," he says. "The government wanted it built in four years but a building that big should have been built in five. You have to control everything: the details, the quality of construction. You have to take a lot of decisions that appear while you're building. The main challenge was to build that quickly while keeping the architectural value of the building."

Mangado's run-in with ETA began while digging up the citadel car park, which was to be the site of the concert hall. The citadel was built in the shape of a five-pointed star (see site map, page 41), but although it is otherwise well-preserved, two points of the star were demolished in the early 20th century to make way for a road. "We knew some foundations of the old walls of the citadel were under the car park, but we never thought they could be in a good state," says Mangado. "When we started the works it was unbelievable, we found wonderful foundations. So I decided to incorporate the ruins into the building."

The foundations of one whole point were incorporated into the walls of the complex, and now form part of the exhibition area. But it was this that gave ETA its chance to attack this high-profile government commission. "ETA hate to see the government succeed," explains Mangado, "so when we found the foundation walls, it tried to stop work."

ETA has particular designs on Navarre, the prosperous region of which Pamplona is the capital and in which Basque culture and language have ancient roots. The group cited Spain's preservation laws and claimed that Mangado's plans to build on the ruins were destructive. "They took me to court and tried to get me a two-year sentence," recalls Mangado, himself from Navarre.

"It was very funny."

But the move seems to have backfired on the separatists, helping to erode what little support the Navarrese had for an autonomous Basque state. As Mangado explains: "The construction of this building became a symbol of hope for the future, like the Guggenheim for Bilbao. It represented freedom and the fact that society, culture and democracy were moving ahead." Mangado laughs at the notion that he might have done time: "I was never worried; I had a wonderful group of lawyers – government lawyers."

Mangado met his four-year deadline, with the concert hall building opening on time in October and only 1% over budget. The design may renounce the spectacular, but its mass and materials give it plenty of presence. Laid out in an L-shape, the building brackets a large public square with open views of the citadel and the town centre. The concert hall name, Baluarte, means "bastion" in Spanish, and it is appropriate for the bulk of the building; but at the same time the hall is self-consciously understated, not distracting too much from its historic neighbour. The smooth facade, made of black granite from South Africa, echoes the solidity of the citadel walls and is only interrupted here and there by carefully placed windows.

"Architecture today is too stylistic," says Mangado. "There is no depth. I always tell my students at the University of Navarre that architecture should try to recover certain ideas, like space, and how to manipulate natural materials and the use of natural and artificial light." In Mangado's own design, all these qualities come to life in the building's interior. In the foyer, for example, the ceiling is punched with circular holes that let in sunlight during the day but also emit artificial light at night.

The walls of the foyer are lined with padouk wood. Mangado came across the material in his modelmaker's workshop, where a red piece sat next to a sample of white wood. He assumed that they were from different species, but his modelmaker explained that they were from the same tree. This particular type of wood, which is native to South America, turns a deeper red the longer it is exposed to air. Since Mangado planned to line the auditoriums with ash, he decided that padouk in the foyer would provide a sensuous contrast. He laid it out in the sun for three months until it oxidised into a charred crimson.

The Baluarte concert hall has been well received. So much so, in fact, that having accomplished his first major project, mastered a demanding schedule and seen off a bunch of litigious terrorists, Mangado now believes he has a genuine cause for concern. "I have had so many congratulations that I was worried about what I must have done wrong. I like polemic. If everyone says you're fantastic, something is wrong."