Years later, in February 1930, the queen recalled with nostalgia the moment when Bran, “the little forgotten fortress beyond the mountains,” entered her life. “Many years ago, on a trip across the border, I saw it standing alone on that rock and wondered what it would be like to own such a fortress and transform it into a home. How wonderful it would be to bring a small medieval castle, a true fairy tale, back to life. And the incredible really happened: two years after the war, the Brașov authorities came to me on a solemn mission and offered me Bran Castle, as an absolute gift, to be mine in its entirety.”

After Queen Marie took possession of the castle between 1920 and 1930, the castle underwent a series of architectural transformations, stemming from her desire to transform it into a modern summer residence. The works were coordinated by the Czech architect Karel Liman, who had also worked on Peleș and Pelișor castles. Two towers were added for additional staircases, the battlements and the portcullises were transformed into windows, and the stoves and fireplaces into modern chimneys. “Bran Castle,” the queen recalled, “meant a newly plowed field, the rebirth of a new vision of beauty. With the help of an old and trusted architect, as enthusiastic as I was, I began to bring life to the cold walls, to revive the old fortress that had never truly lived. I woke it from its long torpor, transforming it from an inert object into a home that looks out at the world through its many windows. Asleep, distant, untouchable as it had been, it would not have imagined that it could be transformed into a warm and welcoming home.”

In order to become the home of the royal family, the castle was equipped with all the necessary facilities for a true royal residence. Water was provided by the well drilled in the castle courtyard, at a depth of 57 m. For lighting, Queen Maria ordered, in May 1932, the construction of a turbine power plant, to which the communities of Bran, Șimon and Moeciu were also connected. These were, as stated in a letter of gratitude sent by the inhabitants of the three villages to the queen, “poor traditional Romanian communities, which could not have received such a benefit in the foreseeable future.” The turbine, manufactured by the Voith company, was put into operation on August 29, 1932. In December of the same year, an 85 HP hydroelectric power plant was built on the Turcu River to illuminate the castle and the surrounding area.

Communication with the outside was ensured by three telephone booths in the castle. The telephone installation and the power plant were modernized and replaced in 1941 and 1944, at the request of Princess Ileana. Since the castle had a ground floor and four floors, it was also equipped with an elevator, like Peleș Castle. This was completed in 1937 to make it easier for the queen to move from the park to the castle. To facilitate communication with the Tea House, a cable car was installed, used to transport food to the traditional five o'clock teas, served by the queen to her guests. The Tea House was equipped with hot and cold water, electricity and sewage, and was arranged for small receptions in a natural setting. In 1946, methane gas was also introduced to the castle.

At Bran, as in other palaces and castles, Queen Maria's architectural dreams were fully realized. Thus, a hunting lodge, a small wooden church, a wooden house with seven rooms, and two huts (one for the queen and the other for Princess Ileana) were built.

Bran Castle was, along with Balchik Palace, the most beloved residence of Queen Maria, who, with her well-known imagination, transformed the solid and seemingly inhospitable fortress into a modern and comfortable summer residence.

The queen's great passion was interior design, and the salons at Pelișor and Cotroceni bear the imprint of her royal aesthetic. Bran was not without such transformations either, although the castle's rigid architecture opposed major modifications to the rooms. As the queen herself stated, after ten years of renovations: "I did nothing to take away its feudal appearance, I did not change the steepness of the stairs, I did not remove the roof from the porches, nor did I straighten the crooked rooms. The doors remained so low that you have to bend down to enter, the walls are several cubits thick, heavy beams cross the vaulted ceilings, and the floors are so numerous that you hardly realize where you are." After all these transformations, Bran Castle had become "a small museum full of rare treasures brought from all countries."

The transformations carried out by Queen Maria at Bran also included the construction of external structures around the castle: the Tea House, made of wooden beams (144 sq m); the Guest House, made of quarry stone (78 sq m); the Princess Ileana's Children's House (43 sq m); the New Private House (176 sq m); the Staff House (378 sq m); horse stables and six garages.

The castle was inhabited only by a few members of the royal family, for whom separate apartments were arranged. Among them was Queen Mary, whose apartment included a hall, a toilet room, two drawing rooms (including the Yellow Room), a bedroom, a bathroom and a dining room, as well as a room where the sovereign had gathered a collection of personal objects (dishes, icons, statues, paintings and books).

The Queen brought along two of her beloved children, Ileana and Nicolae. Prince Nicolae, to whom the Queen intended to bequeath Bran in 1930, received the most elegant apartment in the castle, consisting of a bedroom, a living room, a room called "Princess Mașka", a guest room, the German room, the music room, the toilet room, a bathroom and two terraces. After his return to the throne in 1930, King Carol II, a passionate hunter, also arranged an apartment in the castle (consisting of a bedroom, a living room, a Tyrolean room and a "Doria" dining room), which he used only when he came to hunt in the area.

Being a woman of faith, Queen Maria arranged in Bran, as in every royal residence she lived, a chapel, painted in November 1927 by Arthur Verona (paid 200.000 lei). The chapel, equipped with a carved and gilded wooden altar, an iconostasis, a chandelier, three pews, three candlesticks, two crosses and two icons, was, together with the small wooden church next to the Hunting House, the place where the queen found peace in troubled times.

Queen Maria transformed not only the castle, but also the Bran Castle Park, which was arranged according to her plans and sensibilities. Flowers were the queen's great passion, and all the royal residences had modern, heated greenhouses for wintering. At Bran, a greenhouse and a small rose garden were inaugurated in July 1922, according to the plans of the architect Karel Liman and built by the Klof & Co. company from Codlea. Flower seedlings (roses, dahlias, chrysanthemums) were brought from Codlea or ordered from abroad (Erfurt and Reading, England), including rare species. The head gardener of the castle was Petre Conrad (with a monthly salary of 3.691 lei). Constantin Pamula remained the head of the Bran Royal Park until 1938.

All the flowers, not only those from Bran, but also those from the gardens of the other royal palaces, were purchased from a florist and a retail outlet in Bucharest, located in the "Malaxa" and "Mica" buildings on Calea Victoriei. The Queen loved not only flowers, but also trees and animals. In 1922, forty apple trees, twenty pear trees, fifty cherry trees, twenty-five plum trees and seventy blueberry bushes were brought to Bran Park from the Royal Gardens Division in Cotroceni. In May 1929, after the closure of the farm in Făgăraș, the Administration of Agricultural and Zoological Exploitations in Bucharest sent 1.000 trout (10–15 cm long) to populate the Bran lake. Some were also placed in an aquarium in the castle.

Queen Maria wanted to populate the lake not only with trout, but also with swans. She commissioned Colonel August Spiess, the administrator of the Royal Hunters, to organize, between August 29 and September 3, 1932, an expedition to Lake Sinoe to bring in swans. He received 3.113 lei, money used to "bribe" the guards of the Old Fortress and to pay the border guards who helped him catch the graceful birds. However, the swans caused great problems for the queen, even generating a conflict with the inhabitants of Bran, when one of them was eaten by dogs. Following the incident, several of the castle's men shot pedigree dogs belonging to the locals, who appealed directly to the queen and protested.

The Queen also loved horses, being a passionate rider. She frequently practiced horse riding in the Castle Park and on the riding grounds in Brașov. As the Queen herself confessed, in those moments “the joy of life flows through my veins like wine.” Her passion was noticed by all who knew her. “All her life,” recalled Eugen Buchman, head of the Chancellery of the Cotroceni Palace, “Princess [Maria] was a passionate horsewoman, which is why several thoroughbred horses were kept in the stables, so that she could practice her favorite sport. Every morning she went out riding, either in Bran Park or on the Cotroceni training ground, in which case she was followed by a rider. […] In the cold and snowy seasons, I often saw her riding in Bran Park, wearing a long, dark blue cloak, decorated with silver chevrons, like those worn by the Circassians, and a silver dagger at her belt. Her cheek was ruddy with the cold, and the halo of her golden hair fell from under her black fur hat. She was as beautiful as the daughters of emperors whom I admired in the stories of my childhood.”

During the reign of Queen Maria, Bran Castle experienced a period of glory as a royal residence. The domain of Bran Castle was expanded by the queen by purchasing or transferring the surrounding meadows to her property. In 1934, the queen purchased the Măgura forest. Additional properties belonging to the sovereign were the riding grounds in Brașov.

The total area of ​​the domain, assessed after the queen's death, was 233 acres, to which were added 183 acres of forest (beech and fir), 93 acres of pastures and 2 acres of hayfields.