Kenilworth Castle gardens have undergone a major restoration thanks to high-tech surveying – and a remarkable 16th-century letter

Who writes letters now? Proper letters with descriptions of where they've been, what they've seen, who they've met? Historians of the future are going to have a thin time with the 21st century and its texting and e-mailing. "C U @ 7 – OK?" doesn't give you much to work on. The immediacy of letters is what makes them so appealing: Vita Sackville-West reporting to Harold on a bad day with the roses at Sissinghurst, Keats burbling to his brother about a Lakeland pilgrimage to see Wordsworth. Much depends on the skills of the letter writer of course, and Robert Langham was ideally equipped to give the brilliantly detailed account of the new garden at Kenilworth Castle, which he wrote in 1575.

Langham was a mercer, a trader in textiles, with a very minor position as Keeper of the Council Chamber Door (sounds like something from Blackadder) in the entourage of his patron Robert Dudley, Earl of Leicester. But it was his mercer's eye that made him such a good correspondent. He knew about measuring things; he could look at an arbour or a flower bed and, without whipping out a ruler, give a good account of its height or size. He describes a fountain of white marble in the centre of the Kenilworth garden "cast into an eight-square, reared four feet high". He gives a detailed account of the aviary on the north wall "in height 20 feet, 30 long, and 14 broad" filled, he says with "lively Birds", English, French and Spanish. But he's also used to dealing with pattern and texture and his letter is full of sensuous images. He writes of paths of sand "not light, or too soft", arbours on the terrace filled with the scent of roses, fruit trees "bedecked with apples, pears and ripe cherries".

Langham's letter has been the chief inspiration for the magnificent restoration of Robert Dudley's garden at Kenilworth Castle, recently completed by English Heritage. It cost just over £2m and is worth every penny. Government "initiatives" can waste that much every day and have nothing to show for it. At Kenilworth, you can step straight into the sounds, sights and smells of the 16th century.

Dudley, of course, did it all to impress Queen Elizabeth. He was a favourite, but never quite favourite enough to become her husband. She gave him Kenilworth Castle, though, and made him Earl of Leicester. She also came to stay several times. The garden was made for what turned out to be her last visit and her longest one, a 19-day party that lasted from 9-27 July 1575. "The Lake, the Lodge, the Lord are yours to command" said the mysterious figure who rose from the lake at the entrance to greet the queen on her arrival. It was Dudley's last try at hooking the Queen, but she didn't bite. She kept all his letters, though, including the last one he ever wrote to her, six days before he died. She put it in the treasure box she kept at the side of her bed and it was still there when, 15 years later, she herself died.

You don't need to know any of this to enjoy the recreated garden, but it gives it extra poignancy. You don't need to be an expert in garden history either, to get pleasure from this painstaking period recreation: the strawberry-edged beds, the fabulously scented old roses and the mounds of hollyhocks. All the plants are ones that would have been available to Dudley's gardener, a Frenchman called Adrian (all the smartest people of the Elizabethan age had French gardeners, because they knew much more about growing and training fruit than English gardeners did).

So from the vantage point of the top terrace, you see geometric enclosures edged in strawberry or thrift, and filled with plants known to be available in the 16th century: aromatic herbs and scented flowers such as Sweet Williams, purple rocket, stocks, wallflowers and an incredible array of pinks. Finding the right kind of pinks and carnations has been a problem for John Watkins, head of gardens at English Heritage. Modern breeders had mostly abandoned the old-fashioned flowers with deeply fringed petals and a spicy clove scent.

Cuttings were taken from an old pink growing on the walls of Sherborne Castle and plants grown from seed collected in the Atlas Mountains of Morocco, home of Dianthus caryophyllus. This pink had long been cultivated in Islamic gardens and late in the 15th century came via Constantinople into Italy where it quickly became a favourite. Once the SOS had spread among enthusiasts of pinks and carnations, John Watkins began to get offers of other old varieties and now more than 20 different kinds have been introduced to the Kenilworth garden.

It's been a long haul for English Heritage, who took over responsibility for the castle in 1984. A "Tudor" garden, based on a 1656 map (erroneous as it turned out) had been laid out in the Seventies, with yew topiary and box-edged beds. It was thought to be excellent and won awards. But by the Eighties, the same processes that archaeologists use on ancient sites were beginning to be used in historic gardens. Geophysical surveys revealed the hidden remains of Dudley's garden, deep underground. The Seventies garden was in the wrong place. And research had now shown that topiaried yew wasn't used in Elizabethan gardens. Worse, the roots of the newly introduced yews were breaking up the underground remains of the original garden.

"We had three choices," says John Watkins. "Do nothing and lose the archaeology. Remove the Seventies garden and grass over the whole site. Or make a new, more accurate representation of Dudley's garden." Fortunately, the Wolfson Foundation came up with the money for research to start on the third option and in 2005, archaeologists discovered the foundations of the fountain that Langham had written about, the dimensions exactly as he had described. They found chips of white marble as well.

Having fixed the centre point of the garden, the rest could be laid out, just as Langham had written, with arbours, aviary, obelisks, all superbly detailed. Trellis work of split sweet chestnut lathes surrounds the space, planted with baby hedges of privet and eglantine rose that in time will grow up and completely obscure the woodwork. The aviary has pheasants, guinea fowl and old-fashioned canaries that have settled in so well, they've raised a family. The strawberries, dripping with fruit, seem to have come straight from a 16th-century tapestry. "Delight unto all the senses," said Langham. And now, again, it is.

Kenilworth Castle and Elizabethan gardens are open daily (10am-5pm), £7. See english-heritage.org.uk/kenilworth for further details