Wireless sensor networking, an emerging new field in technology, now opens a variety of application scenarios including remote surveillance, military or home security, disaster warning and so on. Energy efficiency of the network is considered with utmost priority in order to lengthen the lifespan of the network. Here we propose a two layer hierarchical routing protocol called Group-based SEnsor Network (GSEN), which ensures the minimum energy dissipation in the network while introducing only reasonable amount of delay in data delivery. Simulation results show that GSEN performs better than other hierarchical routing protocols such as Low- Energy Adaptive Clustering Hierarchy (LEACH) and Power-Efficient Gathering in Sensor Information System (PEGASIS). In addition, GSEN is completely self-configurable and robust to any changes in network topology.