The symbolism of the gate (standing for conquest) is made clear by the "arms of Spain," and Conrad's imagery continues the message that progress is just another form of conquest, waiting to take its place on the armorial shield. To Decoud, this "inner aspect" of perpetual conquest is clear. But he is still "in his corner," i.e., trapped between the equally abhorrent alternatives of barren skepticism and illusory idealism.

Note the multiple meanings of "device" here: there is "heraldic image," "machine" (referring to material progress), and "literary symbol" (which the archway is).