The original layout of this Jacobean building echoed medieval designs. Changing fashions have influenced its subsequent evolution. In medieval times the Hall was the largest room and focus of the house. On important occasions everyone ate there; the owner seated on a dais at the head of the room and others ranged below in order of rank. A doorway led from the Hall to the owners private apartments.
By Jacobean times great events rarely took place in the Hall. Instead guests were entertained in the Grand Chamber (the Great Dining Room) which like the other private rooms had increased in size and importance. At Lulworth, the Hall remained an imposing room, leading via an arch to the main stairs and upwards to the Great Chamber. Beyond this were the State Apartments occupied by the persons of highest rank present and entered by others only at their invitation. The second floor was divided into four slightly less important suites, each of three rooms. Above, viewing parties could survey the panorama of the surrounding countryside from the lead roof or retreat into rooms at the tops of the corner towers.
After the Civil War Humphrey Weld needed to refurbish the Castle's interior. On the first floor doorways were realigned 'en filade' in the French fashion. On the second floor the corner rooms were divided up into small rooms and corridors which led to the towers, creating seven smaller suites each with a bedchamber and a closet.
In the 18th Century Edward Weld modernised many rooms in the Castle. A new and sophisticated reorganisation was planned by his son Edward, the next owner, but never implemented due to his untimely death. Instead his younger brother Thomas inherited the estate and engaged the architect John Tasker to design an alternative scheme. The terrace was rebuilt as a hollow structure adding extra service rooms to the basement. Within the main building alterations included the enlargement of the chapel, replacement of the main stair, creation of a larger entrance hall and redecoration of the principal ground and first floor rooms in the fashionable Neo-Classical style. Soon after the completion of the work, Thomas Weld again used John Tasker to build the chapel in the grounds.
The last significant re-styling was carried out in the mid 1860's, using the architect Joseph Hansom. He created a corridor directly linking the two ground floor entrances so that visitors entering the front door no longer had to circulate through the main rooms to reach the rest of the Castle. The newly isolated South East corner of the building became the 'gentlemen's area', containing the Billiard Room and the owner's Study. Elsewhere in the Castle comfort was improved by adding central heating and more fireplaces.