Example 4.5.6.9 (The Postnikov Tower). Let $X$ be a Kan complex. Then the tower of fundamental $n$-groupoids
\[ \cdots \rightarrow \pi _{\leq 3}(X) \rightarrow \pi _{\leq 2}(X) \rightarrow \pi _{\leq 1}(X) \rightarrow \pi _0(X) \]
is an isofibrant diagram of Kan complexes (Corollary 3.5.8.9).