unit ball in $\mathbb C_p$ as an inverse limit

Question

I'm reading Robert's "A Course in $p$-adic Analysis". The Theorem at the bottom of page 79 says that there is an isomorphism $A \cong \varprojlim A/\xi^nA$. ($A$ is the unit ball and $\xi$ is an element with $\vert \xi \vert < 1$.)

Robert p. 79

Is the mentioned isomorphism an isomorphism of rings or of topological rings? What is the "correct" topology on $A/\xi^nA$?