"You're living in listed accommodation, writing for orchestra," says Thomas Adès, "or putting on someone else's clothes and feeling absolutely new yourself". Prior to Asyla, Adès had written only two works with a symphonic dimension : the Chamber Symphony (1992), and the spectacular Living Toys (1993) for chamber orchestra, which is in essence a short tone-poem. Asyla shares two elements crucial to traditional symphonic form: it is in four movements, and it uses the symphony orchestra. It is the work of Adès that most closely resembles a symphony.

But here the similarity ends, for if we try too hard to match Asyla to the idea of the Romantic and Classical symphony, we miss the point. This is not to say that these forms do not cast a far-reaching shadow; they do, as does the traditional orchestra which developed alongside them. (Composers only recently have begun to step out of those shadows.) Adès describes the orchestra as the most durable and vivid medium for which to write. But it is no longer a mainstream medium; composers have evolved, whereas the orchestra has been frozen in a pre-First World War state.

Asyla is an exploration of the pull between the safety of tradition, and daunting freedom. The title, the plural of asylum, is deliberately ambiguous. It refers both to madhouses and to sanctuaries (political asylum, for example). Reflecting these themes, the first movement evokes a sense of motion across open spaces, the inner two movements take place as if in an enclosed setting, and the finale bursts these confines to provide a final, unexpected release.

As with all of Adès's orchestral music, the most striking element is the wildly imaginative way he uses the traditional symphony orchestra. For example, the percussion in Asyla is written mainly for metallic instruments. Even the timpani are struck on their metal shells at times. These instruments played this way generate a shimmering surface. Amid the percussion, there is an upright piano tuned a quarter of a tone lower than the rest of the orchestra. It provides an uneasy glow to every texture in which it plays.

The first movement of Asyla describes a large arch, with an introduction (cowbells and gongs predominating) and a long melody begun by the horns. A violent middle section, from which the strings are largely excluded, is followed by a return both of this melody and of the introduction, the two drastically condensed. The second movement, originally titled Vatican, moves from this sense of the outdoors to a vast enclosed space. A dark, musky interior is plaintively described by the long falling tune of the bass oboe. At the mid-point of this movement, the violins play a long pianissimo descending sequence, with the unusual sonority of added practice mutes. The melody is in E flat minor, Asyla's tonal centre of gravity. This sonority is revisited at the climax of the last movement.

Ecstasio, the third movement, is a dance, with a similar function to a Scherzo in a Romantic symphony. It is inspired by the insistent rhythms of club music and has a very primeval feel, gradually building from short and melancholy chord progressions, to a thrashing tutti, and beyond.

The last movement, beginning with a deceptive simplicity, nevertheless sums up what has gone before. Its middle section superimposes melodies from the first and third movements upon distant harmonies from the second; beneath, a canopy of woodwind and cowbells suggest open spaces somewhere close by. The climax (again E flat minor) first anticipated in the out-of-tune piano, is violently corrected by the orchestra; then the scene is flooded with light and there is a sudden sense of release, recalling the opening of the work.

The four movements are played with only brief pauses between each, giving a sense of continuity. Asyla is an utterly fluid work, yet it is built upon a rigorous structural integrity. As with all Adès's works, the instructions to the players are very detailed, and the effect is one of a highly wrought, tightly controlled, but infinitely flexible orchestral canvas.