the first room itself is very tiny, a pathetic little maze with corpses, robbers, and racers for some reason. until, The Genjin leads you to the stomach of a king??? its the stomach of something idk i didnt get that part. the stomach is the first area x10, a confusing labrynth with a subtle overarching story of dr. ryuchi, a mad scientist with an odd obsession for kings which led to him, and the other scientists, eventual deaths. you can meet ryuchi's brain and he calls you a tremendous pussy if you don't do what he asks.
the altar can be found by continuing into the stomach until you enter the first large room. from there, turn left and keep on going forward, you'll eventually see it!