How to draw a contour diagram in Mathematica?

Here's a quick 5 minute reproduction of the diagram (I've left the labeling of individual points out):

With[{ε = 0.05, L = 1, thick = AbsoluteThickness[1]}, 
    Graphics[{
        thick, 
        Circle[{0, 0}, ε, {π/2, 3 π/2}], 
        Arrowheads[{{0.05, 0.99}}], 
        Arrow[{{0, -ε}, {L, -ε}, {L, -L}, {-L, -L}, {-L, L}, {L, L}, {L, ε}, {0, ε}}], 
        Text[Style["\!\(\*SubscriptBox[\(C\), \(n\)]\)", FontSize -> 15], {L/2, 0.1}]
    }, Axes -> True, AxesStyle -> thick, Ticks -> False]
]

The key points to make life simple are:

• Use a single Arrow chain for the entire length.
• Use the appropriate definition of Arrowheads to specify the position of the arrow head.
• Use AbsoluteThickness to get a uniform thickness.
• Use the 3 argument form of Circle to draw arcs.

Graphics[{Circle[{0, 0}, 1, Pi/2 {1, 3}], 
           Arrow[{{10, 1}, {0, 1}}], 
            Line[{{0, 1}, {10, 1}, {10, 10}, {-10, 10}, {-10, -10}, {10, -10}, {10, -1}, {0, -1}}]},
         Axes -> True, Ticks -> None]