A person familiar with the investigation told Reuters the 13 people in question were linked to the Marzotto group, and included members of the Marzotto family.
Marzotto sold Valentino Fashion Group - then including both the Valentino label and Hugo Boss - to private finance group Permira in 2007.
The assets, including apartments in Milan and Rome, a 25-room villa in Alpine resort Cortina d’Ampezzo and land, were taken preventively to cover €65m in taxes the 13 people are suspecting of having dodged when they booked a €200m capital gain from selling Valentino, reports the Financial Times.
The Marzotto family members avoided Italian tax obligations by using a Luxembourg-based holding company for the Valentino sale.
Those under investigation are suspected of not having filed tax returns.
A spokesperson for the Marzotto Group declined to comment, saying the news did not involve the company or any of its units.
The Italian government has set fighting chronic tax evasion as one of its priorities as it seeks to come to grips with the country's towering debt crisis and find resources to fund growth.
Lawyers representing the Marzotto family said the decision taken by Milan prosecutors ordering the seizure was "totally groundless".
The lawyers said bank documents showed capital gains from the operation had been declared and taxed.
"I acknowledge the seizure measures. I think it right only to point out that I did not have any operative position in the company in which I was minority partner," Matteo Marzotto, a board member of the textile family group, said in a statement.
The seized castle, Villa Trissino Marzotto, is near the town of Vicenza and has been in the Marzotto family for 60 years. With more than 50 rooms, it was expanded in the 18th century and has on its grounds two Italian-style geometrical gardens, a small forest, a road lined with lemon trees and more than 100 statues, reports the Financial Times.
The technocrat government of Prime Minister Mario Monti has described the fight against tax evasion as a state of war and has stepped up monitoring and collection efforts.
Earlier this year the head of Italy's Inland Revenue service, Attilio Befera, said tax evasion totalled some €120bn.
"The investigation... revealed that a financial holding company purposefully created in Luxembourg was instead administered from Italy," the police said.