deep dark tunnel.

its a few hundreds long and its dark and damp.
this is where the narrow boats plod back and to from Chirk to Llangollen.
nowerdays they have engines to drive them through but in the victorian times when the horses refused to go through the boat owner would have to lay on his back on top of the boat and "foot" through by walking and pushing with his feet on the roof of the tunnel.
like a river the canal has a flow of water which strengthens at the narrow parts of the canal, so in a tunnel like this it would be a damn site harder going one way than the other.
the horse would be taken over the canal via a towpath