More than 200 ancient items and 300 paintings were found inside sealed containers in a royal stable and in the basement of the main residence at Tatoi, some 25 kilometres (15 miles) northwest of the Greek capital, culture ministry officials said during a media tour of the site on Tuesday.
"It's a real treasure hunt, we are in the process of removing these marvellous items from boxes stacked in disorderly heaps," restoration supervisor Nikos Minos told AFP.
A team of 21 archaeologists and restorers started work at the crumbling, 19th-century estate three months ago as part of a bid to catalogue its contents before restoration work starts to find a new role for the site.
The collection includes the bronze helmet of an ancient Greek soldier, ancient glasswork including a perfume vial from Roman times, idols and clay vessels -- among them a 2,700-year-old painted jug bearing the form of a horseman, found intact to the amazement of archaeologists.
"It is a collection of great value, and happily in good condition despite the storage conditions," said archaeologist Dimitris Kaziannis.
The paintings are mainly 19th-century Greek masters but a number belong to the 18th-century French and Venetian schools.
The search also yielded over 100 religious icons and vessels, most of them not known to be in the royal collection catalogued by the late Queen Frederika, mother of Constantine and Sofia, the present-day queen of Spain.
Kaziannis said archaeologists found "a lot more" than was listed in either the inventory compiled on Frederika's orders, a later catalogue drawn up in 1973, or a final tally drawn up in 1991 when the former royal family was permitted to remove a number of items inside 10 containers.
"We are continuing our search, there are surely more items to discover," said Kaziannis.
Home to the Greek royal family for decades, Tatoi in its heyday welcomed the cream of European royalty, from Kaiser William II of Germany and Tsar Nicholas II of Russia to King Edward VII of England and Empress Elisabeth (Sissy) of Austria-Hungary.
Originally purchased in 1872 by King George I of Greece, a scion of Denmark's ruling house of Schleswig-Holstein-Sonderburg-Gluecksburg, Tatoi fell onto hard times when its last owners abandoned it in 1967 shortly after a group of army putschists took control of government.
The estate was seized by the junta in 1973 along with other properties, sparking a legal grudge that was only resolved in 2002 when the European Court of Human Rights ordered the Greek state to pay 13.2 million euros (17.6 million dollars) to the former royal family.
Today, the rustic 4,700-hectare (11,610-acre) estate lies largely abandoned, its 37 romantic-style buildings in various degrees of disrepair and some barely standing.
Greek Culture Minister George Voulgarakis on Tuesday said the Greek state intends to turn Tatoi into a park and establish a museum on the site, but the restoration effort will require at least another five years.
"Whether we like it or not, these items and buildings are part of our history and they should be exhibited," he told reporters.
The Greeks voted to abolish the monarchy in 1974, in a referendum held shortly after the army junta collapsed and democracy was restored in the country.
In January, Greece unsuccessfully tried to halt the auction at Christie's of 850 objects belonging to George I -- many of them silverware and works by jeweller Peter Carl Faberge.