Mesoa was one of the 5 villages that founded Sparta. The name of the 5th village is unknown, however, more recent claims have been made saying it's Amyklai. The villages were located along the banks of the Eurotas River, and became unified around 775 B.C.

The tomb of King Leonidas I is the only public structure that remains (partially) in Mesoa today. It was a carved lion monument bearing the following inscription testifying to Leonidas' bravery in the Battle of Thermopylae:

Go, tell the Spartans, stranger passing by,
That here, obedient to their laws, we lie.

King Leonidas died, out-numbered, in the Battle of Thermopylae. According to Herodotus, the Persian army went to battle with 2,317,610 men, while the Greek army had 5,200 (300 Spartan).

Before the battle, it is said that Sparta consulted the Oracle at Delphi. The Oracle's warning said that either Sparta would be conquered and left in ruins, or one of her two kings must sacrifice his life to save her. This is possibly the reason Leonidas went to battle with so few men.

Prophecy of the Oracle at Delphi to Sparta:
O ye men who dwell in the streets of broad Lacedaemon!
Either your glorious town shall be sacked by the children of Perseus,
Or, in exchange, must all through the whole Laconian country
Mourn for the loss of a king, descendant of great Heracles.
He cannot be withstood by the courage of bulls nor of lions,
Strive as they may; he is mighty as Jove; there is naught that shall stay him,
Till he have got for his prey your king, or your glorious city.

Source Material:
Ancient Greece
Wikipedia