How much of the work was done by Primaticcio himself is hard to establish Certainly the original plan or sketches were his and the master and assistants who worked on it were under his control, so it is attributed to him. It is more elaborate than others on the subject before him.
The small chapel of 1250 is a jewel! It is of course modelled after St.-Chapelle. It was redecorated in the 1550's by the commendatory abbot Ippolito d'Este who had more influence (and some money) than anyone in Europe. He was able to get Primaticcio and his team to come over from Fontainebleau and do the design and work. Over the inner face of the West wall is an Annunciation fresco more elaborate than most created before it.(Shown in a separate Tip). Moreover, we thought that what we saw on the vaults is finer mural work than Fontainebleau. The facade was improved by the crossed ogival window and doorway. There are fine gargoyles and carved figures. Of course Primaticcio had seen the Sistine chapel and so he covered the ceiling with Apostles, Evangelists and Fathers of the Church (obtain a diagram-scorecard) and capped it off at the east end with Angels carrying the Instruments of the Passion.It is certain that he only did the sketches and iconography. His assistants did the "work". The walls are covered by a trompe-l'oeil curtain hung with the coats of arms of the Abbots of the Abbey of Chaalis. This appears to have been added by Madam Vatry the first 19C owner and builder of the chateau
All of the finest paintings are in one room called the (picture) Gallery. They are mixed in with rare furniture, statuary and a large mantel topped by a life-size carved stag. The only real lighting is from outside. This is not an "Art Gallery" but a room to meditate in, and you can get clos to some of the works. There are clarifying placards with some of the works like in a real museum but there without a guide you would not know where or what to look at next. (Since we were there they may have produced some literature for sale, even an English version). You are no longer permitted to take photographs, their website is almost devoid of illustrations and I lost any notes I may have made, so the identification of some painting may be wrong or to start with as doubtful (marked ?). There is no missing Giotto or Signorelli. We are at a total loss at identifying the fine statuary (that are not in the Bust Gallery) and have made them into a Travelog if you want to take a look (not done yet).