Straight Edges

The fortress emerged ignorantly from the earth. Blocky towers and walls squared the cliffside; the natural terrain began and ended abruptly and untouched by the builders. Even Lake [cool name], a huge thing with bigger boats than I know the names for, was no impediment. Its dark icy surface refected the well-cut stone and continued the Pathagorian line started high above my position. If this castle rose from depths; what might it contain?