Commutative Diagram (Homological Algebra)

Question

I am having a hard time with the following commutative diagram. Mostly, I am not sure how to align the middle row correctly. Currently this is my best attempt.

I cannot get the code to compile upon deleting that unwanted $N_n$ (this is only a sort of dummy so that the code will compile). Also, I can't get the diagram to align nicely as in the picture.

\documentclass[border=10pt,tikz]{standalone}
\usepackage{tikz-cd}
\begin{document}
 \begin{tikzcd} 
 C_{n+1} \arrow{r} \arrow{dr} & B_n \arrow{d} \arrow{r} & C_n \arrow{r} \arrow{d} &  B_{n-1}  \\ 
 &  A_n \arrow{r} \arrow{d} & A_{n-1} \arrow{r} & N_n  \\
E_{n+1} \arrow{r}  &  D_n \arrow{r} & E_n \arrow{r} &  D_{n-1}    
 \end{tikzcd}
\end{document}

Answer 1

\arrow{d}

means go one-below. So there must be something there.

\arrow{dd}

means go two-below.

A cell cannot start with an arrow and then have something in it. So

\arrow{r} C

is no good but

\arrow{r} & C

is fine.

Complete code:

\documentclass[border=10pt,tikz]{standalone}
\usepackage{tikz-cd}
\begin{document}
\begin{tikzcd}
  \arrow{r} & C_{n+1} \arrow{dd} \arrow{dr} & & B_n \arrow{dd} \arrow{r} & C_n \arrow{dd} \arrow{dr} & & B_{n-1} \arrow{r} \arrow{dd} & {}\\
  & & A_n \arrow{ur} \arrow{dr} & & & A_{n-1} \arrow{ur} \arrow{dr} \\
  \arrow{r} & E_{n+1} \arrow{ur} & &  D_n \arrow{r} & E_n \arrow{ur} & & D_{n-1} \arrow{r} & {}
\end{tikzcd}
\end{document}