The settlement of Thermi was built about 3000 BC on a flat peninsula on the eastern coast of Lesbos, in one of the most fertile areas of the island. The location of the settlement was favoured by the development of agrarian economy and marine trade which made Thermi one of the earliest and most significant early urban centres of the north Aegean during the first half of the 3rd millenium BC. The intensive habitation of Thermi led to its expansion on an area of 1,5 hectare and imposed from the beginning an organized town planning to cover the needs of sheltering a constantly growing population. Seven building phases of the Early Bronze Age (Thermi I, II, IIIA-IΙIB, ΙVA-IVB and V) are distinguished at a deposit of about 6 metres thick; these have been searched at an area of 3000-6000 square metres. The preserved remains of the Middle and Late Bronze Age are very few while part of the settlement has been destroyed by the erosion of the sea.
The rectangular long and narrow building (megaroid) composed of the closed and less frequently open to the road antechamber and of the main chamber is the basic building unit. Its length varies from 13 to 17 metres and it is 3-4 metres wide. They are disposed one next to the other with entirely common stone-built long walls forming building blocks. Three different architectural plannings which always correspond to the number of the inhabitants and aim at the best possible communication of the insulae are discerned in the settlement history of Thermi. In Thermi I-IIIB a central building group and other ones developing in a radiating settlement plan around the centre are distinguished. Roads 2 metres wide separate the building groups and lead from the circuit to the centre of the settlement. This is the closed "radiating" building system, known in contemporary with Thermi settlements of western Asia Minor (Troy, Besik-Tepe, Bakla Tepe, Demircihuyuk, etc.). In phase IIIB the central part of the settlement is protected by a stone fortification wall 1-2 metres thick, reinforced with at least 4 trapezoid towers.
In phase IVA, the settlement of Thermi is completely reorganized and in phase IVB it includes large building blocks of the same size which are separated by parallel, in part stone-paved roads 2-5 metres wide. This is the "rectangular" settlement plan which occurs for the first time in the architecture of the prehistoric Aegean to acquire its idealized form during the historic period in the Hippodamus system. In phase V, the planning of Thermi is defined by the opening of a central road (1-2,5 metres wide) which aims at the better communication between the building blocks. The new settlement planning which was previously applied in Poliochni, is named "linear" and is characterized of the presence of basic communal functions (building Theta 1, square N2) and manufacturing ones (metalworker's) on the central road of the settlement. In this phase the settlement has about 1200 persons and is protected by a complex defensive system which is formed in phase IV. It includes a strong internal wall (2 metres thick, preserved at 1,5 high) and two stone fortification walls that are parallel to the internal one. Cross walls connect the internal wall with the first fortification wall forming small rooms suitable for living and storing. Two gates, fortified with projecting towers serve the entrance from the inland to the interior of the settlement.
The settlement plan and the re-planning of Thermi, the stone-paved roads, the distinguished fortification system, the presence of many storerooms in each household (pithoi, stone platforms, storage pits), the existence of metalworking workshops (area E), the treasuring up of bronze tools (Thermi IVB-Potter's pool), bronze and silver jewellery and other rare objects in the Aegean (bracelets made of lead and tin in Thermi IIIA) allow the determination of Thermi as an early urban centre with developed politics and economic organization and social composition. It appears that this powerful settlement did not endure the economic competition of early urban centres such as Poliochni, Troy or Liman Tepe around 2500 BC and was abandoned. Its inhabitants must have sought for safety in the inland of the island.