% Code from Christian Feuersänger
% http://tex.stackexchange.com/questions/54794/using-a-pgfplots-style-legend-in-a-plain-old-tikzpicture#54834
% argument #1: any options
\newenvironment{customlegend}[1][]{%
    \begingroup
    % inits/clears the lists (which might be populated from previous
    % axes):
    \csname pgfplots@init@cleared@structures\endcsname
    \pgfplotsset{#1}%
}{%
    % draws the legend:
    \csname pgfplots@createlegend\endcsname
    \endgroup
}%

% makes \addlegendimage available (typically only available within an
% axis environment):
\def\addlegendimage{\csname pgfplots@addlegendimage\endcsname}

%%--------------------------------

% definition to insert numbers
\pgfkeys{/pgfplots/number in legend/.style={%
        /pgfplots/legend image code/.code={%
            \node at (0.295,-0.0225){#1};
        },%
    },
}
\begin{tikzpicture}
    \draw[draw=white,pattern=north west lines, pattern color=blue] (-1.5,-1.5) rectangle (1.5,1.5);
    \draw[fill=white] (0cm,0cm) circle(1cm);
    \draw[fill=white,thick,pattern=dots, pattern color=red] (0cm,0cm) circle(1cm);

    \begin{customlegend}[
    legend entries={ % <= in the following there are the entries
    au{\ss}en,
    innen,
    Jordankurve
    },
    legend style={at={(4.5,1.5)},font=\footnotesize}] % <= to define position and font legend
    % the following are the "images" and numbers in the legend
        \addlegendimage{area legend,pattern=north west lines, pattern color=blue,draw=white}
        \addlegendimage{area legend,pattern=dots, pattern color=red,draw=white}
        \addlegendimage{thick}
    \end{customlegend}
\end{tikzpicture}