We study the problem of assigning clusterheads in a hierarchical Wireless Sensor Network (WSN). That is, for a given hierarchical WSN, how many clusterhead nodes we should assign, and how to geographically allocate these clusterheads. Since an assignment scheme optimizing all factors is impossible, we will focus on the crucial issue of energy efficiency of the WSN. Because it is mostly true that the nodes of WSN are powered by batteries, power saving is an especially important consideration in WSN architecture design. We will propose a hierarchical WSN architecture toward the end of saving energy of both sensor nodes and clusterheads. Using analytical result, experiments are conducted in which realistic scenarios are simulated.