I thought I knew Brussels pretty well, but until last year I'd never been around for the annual opening of the greenhouses at Laeken. These huge, circular glass and iron structures stand 5km from the city centre, in Brussels' northern suburbs.

Built in the late 19th century by Alphonse Balat and his apprentice Victor Horta (before he became the king of Belgian Art Nouveau) they were a late addition to the Saxe-Coburgs' royal palace. The family always preferred their home on this wooded hillside to the Palais Royal down in the city. The palace itself is strictly private, but once a year, usually in April, the king opens his glasshouses, Les Serres Royales, for two or three weeks.

Coach parties come from across Belgium and from Germany, France and Holland. The appeal of Laeken is not just this sequence of vast interconnected conservatories and all the flowers contained within, it's the chance to get a glimpse into the home life of the Belgian monarchy.

Turn up on a Sunday, as I did, and the police will be out in force directing coaches up the long avenue that leads to the monument commemorating Leopold, first king of the Belgians. Half the visitors were, I'm sure, playing a royal version of Through the Keyhole.

Royal-watchers will be disappointed, however. The glasshouses provide a sealed off eastern wing to the palace, which sits with its back to the main road. The building itself gives nothing away, not even a flag. As I queued and shuffled, I tried to grasp the evident excitement of the crowd in front of me: families with pushchairs, retirees with video cameras. This was a holiday atmosphere.

Once through the glass doors we all entered a large empty orangery that was built for the Dutch King William, who briefly absorbed Belgium into his United Kingdom of the Netherlands in 1815 after the Congress of Vienna.

The Belgians did not take well to that and a riot broke out in the opera house down below in Brussels. Soon the crown was offered to the Saxe-Coburgs if they would agree to be the first kings of the Belgians, not of Belgium. Even today no one succeeds to the throne until he or she first swears an oath: "I swear to observe the laws of the Belgian people."

In 1874, royal favourite Alphonse Balat was instructed by Leopold II to integrate the old orangery into a sequence of new glasshouses that would rival Cousin Albert's Crystal Palace in London. As we passed from the older structure to the new one, iron and glass seemed to soar high above us, bringing light flooding in on all sides. Surmounted by a dome-like crown and vented by two tall chimneys built to resemble minarets, the central structure looks like a glass-and-metal mosque. It was while working on this project that a young Victor Horta worked out what these materials could bring to Art Nouveau.

Our path through the first glass house was clearly marked and bordered on all sides by giant palms, orange trees, camellias and geraniums. In the centre beneath the dome was an open circular space surrounded by tall carved columns. When the Serres Royales were finally completed, Leopold II was so delighted with this part of the structure that he moved the royal chapel in. Today this central dome is still known as the Iron Church.

The tourist route is circular. But because Balat and Horta built only a three-sided structure, that means visitors have to step outside to walk the fourth side along gravel paths overlooking the royal gardens. This area was once a hunting ground for the dukes of Brabant, whose lands included the medieval city of Brussels. Now the landscape is tamed, a rolling Arcadian paradise with a tree-lined lake, classical statues and rhododendrons. There's even a Japanese pagoda rising on the horizon.

This is the kind of luxury that only a rich African colony can buy you. Leopold II, who employed Balat and Horta on these greenhouses for 21 years, drew immense personal wealth by ruthlessly exploiting the Belgian Congo – and it shows.

The size of this whole enterprise is huge. It takes 20 full-time gardeners to keep the plants in order and 400 tons of fuel to sustain the right temperatures inside over the course of a year. Washing the windows must be a major undertaking, and after over a century of use, there's rust forming on the iron girders.

Our trail ended back in the orangery. It was here that the wedding reception of the Belgian Princess Astrid was held in 1984. Again, royal-watchers will be disappointed: no wedding photos are on display and you can only buy postcards of the greenhouses and the plants. I got the feeling that the Saxe-Coburgs don't market themselves like their English cousins. This monarchy keeps it head down, especially for those three weeks a year when the hoi polloi are let in.

The Royal Greenhouses at Laeken (00 32 2 551 20 20; tinyurl.com/czrtwk) are open from 18 April-10 May 2009 (excluding Mondays). Admission on 21 April is reserved for disabled/less able visitors only. Admission €2.50.