N-level nested nodes

Question

I read this question but I didn't get how to repeat the process more than one time. I make an example: I'd like to have a big green box which contains two copies of the light blue box presented in the other question. It, again, contains the blue boxes with the numbers.

I'm starting with this

\documentclass[10pt]{article}
\usepackage{tikz}
\usetikzlibrary{backgrounds,fit}

\begin{document}

\begin{tikzpicture}[outer sep=0.05cm,node distance=0.8cm,]
\tikzstyle{bigbox} = [draw=blue!50, thick, fill=blue!10, rounded corners, rectangle]
\tikzstyle{box} = [minimum size=0.6cm, rounded corners,rectangle, fill=blue!50]
%
\node[box] (11) {1};
\node[box,right of=11] (12) {2};
\node[box,right of=12] (13) {3};
\node[box,below of=11] (21) {4};
\node[box,right of=21] (22) {5};
\node[box,right of=22] (23) {6};
%
\begin{pgfonlayer}{background}
  \node[bigbox] [fit = (11) (23)] {};
\end{pgfonlayer}
%
\end{tikzpicture}

\end{document}

Answer 1

Easy job with matrix library:

\documentclass[10pt]{article}
\usepackage{tikz}
\usetikzlibrary{backgrounds,fit,matrix}
\tikzset{
   bigbox/.style = {draw=blue!50, thick, fill=blue!10, rounded corners, rectangle},
   box/.style = {minimum size=0.6cm, rounded corners,rectangle, fill=blue!50},
}


\begin{document}

\begin{tikzpicture}[outer sep=0.05cm,node distance=0.8cm]
   \matrix (a)[row sep=2mm, column sep=2mm, inner sep=2mm, bigbox, matrix of nodes, every node/.style=box]
      {
        1 & 2 & 3\\
        4 & 5 & 6\\
        };
\matrix (b)[row sep=2mm, column sep=2mm, inner sep=2mm, bigbox, matrix of nodes,
        every node/.style=box,anchor=west] at (a.east) {
            1 & 2 & 3\\
            4 & 5 & 6\\
        };
%
    \begin{pgfonlayer}{background}
        \node[bigbox,draw=green,fill=green!10]  [fit = (a) (b)] (A)  {};
    \end{pgfonlayer}
%
\end{tikzpicture}

\end{document}

If you want to increase the sepration between two blue boxes use this:

\matrix (b)[row sep=2mm, column sep=2mm, inner sep=2mm, bigbox, matrix of nodes,
        every node/.style=box] at ([xshift=2cm]a.east) {
            1 & 2 & 3\\
            4 & 5 & 6\\
        };

Change xshift suitably.

Answer 2

As Harish explained, a matrix offers two layers (inside nodes and matrix) but both are drawn on main (default) layer. backgrounds library introduces two more layers background and foreground. But if you need more layers you can define them with \pgfdeclarelayer and once declared order them with \pgfsetlayers command.

Next code based on Harish example shows how to use both commands.

\documentclass[10pt]{article}
\usepackage{tikz}
\usetikzlibrary{fit,matrix}
\tikzset{
   bigbox/.style = {draw=blue!50, thick, fill=blue!10, rounded corners, rectangle},
   box/.style = {minimum size=0.6cm, rounded corners,rectangle, fill=blue!50},
}

\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfdeclarelayer{minusone}
\pgfdeclarelayer{minustwo}
\pgfdeclarelayer{minusthree}
\pgfsetlayers{minusthree,minustwo,minusone,background,main,foreground}

\begin{document}

\begin{tikzpicture}[outer sep=0.05cm,node distance=0.8cm]
   \matrix (a)[row sep=2mm, column sep=2mm, inner sep=2mm, bigbox, matrix of nodes, every node/.style=box]
      {
        1 & 2 & 3\\
        4 & 5 & 6\\
        };
\matrix (b)[row sep=2mm, column sep=2mm, inner sep=2mm, bigbox, matrix of nodes,
        every node/.style=box,anchor=west] at ([xshift=2cm]a.east) {
            1 & 2 & 3\\
            4 & 5 & 6\\
        };
%

    \begin{pgfonlayer}{background}
        \node[bigbox,draw=red,fill=red!10]  [fit = (a)] (A)  {};
        \node[bigbox,draw=red,fill=red!10]  [fit = (b)] (B)  {};
    \end{pgfonlayer}

        \begin{pgfonlayer}{minusone}
        \node[bigbox,draw=green,fill=green!10]  [fit = (A) (B)] (AB)  {};
    \end{pgfonlayer}
%

        \begin{pgfonlayer}{minustwo}
        \node[bigbox,draw=purple,fill=purple!10]  [fit = (AB)] (AB-1)  {};
    \end{pgfonlayer}

        \begin{pgfonlayer}{foreground}
        \node[bigbox,draw=orange,fill=orange!10,opacity=.5]  [fit = (a-2-2.west|-AB-1.south) (b-1-2.east|-AB-1.north)] (AB-2)  {};
    \end{pgfonlayer}

        \begin{pgfonlayer}{minusthree}
        \node[bigbox,draw=brown,fill=brown!10]  [fit = (AB-2) (AB-1)] (AB-3)  {};
    \end{pgfonlayer}

\end{tikzpicture}

\end{document}