Rosalind and Celia’s conversation about Orlando is interrupted by the arrival of Duke Frederick (at line 30). The moody Duke has a mercurial (inconstant or easily changeable) nature. Previously he had welcomed Rosalind into his court as if she were another daughter to him. But now he is worried because the people of the dukedom admire her and pity her. Duke Frederick is worried that the people under his rule will rebel and oust him just as he had ousted Duke Senior. So, Duke Frederick tells Rosalind that she is banished and that if she is still in the dukedom in ten days, then she will be executed.
Celia attempts to defend Rosalind, but Duke Frederick’s mind is set. Further, he tells his daughter that Rosalind is subtle and cunning (at line 71). He is telling Celia that Rosalind is merely using her so that she manipulate her later and grab the seat of power for herself. Actually, Duke Frederick is describing himself. He is subtle and cunning, and in his insecurity he is projecting his own qualities onto Rosalind. He imagines that others are like himself.
After Duke Frederick exits, Celia shows her fondness and loyalty to Rosalind by declaring that she will leave the palace as well and go wherever Rosalind goes. Celia also suggests that the two of them should seek Duke Senior in the Forest of Ardenne. The two young ladies then decide to disguise themselves as commoners so that Duke Frederick will not be able to know of their sudden departure together.