Zevachim 26 - Position of Sacrifices
Each sacrifice had its prescribed place of slaughter. For example, the most holy offerings were slaughtered in the northern part of the Temple courtyard. If the Kohen stood in the southern part, stretched his arm into the northern part and performed the slaughter, it is valid. On the other hand, if he received the blood while standing in the southern part, the sacrifice is invalid.
The father of Shmuel asked his son a question, "If one suspended an offering in the air and slaughtered it, what is the law?" Shmuel said that it was valid. "Wrong!" - said the father, since the slaughter has to be "At the side" of the Altar, not above. Next question, "What if the slaughterer was suspended in the air?" "It is invalid!" - answered Shmuel. "Wrong!" - said the father, slaughter should be "At the side," not that the slaughterer should be "at the side."
The Altar was 10 cubits (20 feet) high, and a red line marked the separation between the upper and the lower halves. The blood of most offerings was applied to the lower half; only the blood of the sin-offering and of the bird burnt-offering was applied at the upper half - ideally at the "horns" on top.
Art: Giovanni Martinelli - The Sacrifice Of Noah