Walking in Ancient Greece (Education in Sparta) - Assasin's Creed Odyssey

The Spartan education process known as the agoge was essential for full citizenship. However, usually the only boys eligible for the agoge were Spartiates, those who could trace their ancestry to the original inhabitants of the city.
There were two exceptions. trophimoi or "foster sons" were foreign students invited to study. The Athenian general Xenophon, for example, sent his two sons to Sparta as trophimoi. Also, the son of a helot could be enrolled as a syntrophos if a Spartiate formally adopted him and paid his way; if he did exceptionally well in training, he might be sponsored to become a Spartiate. Spartans who could not afford to pay the expenses of the agoge could lose their citizenship.
These laws meant that Sparta could not readily replace citizens lost in battle or otherwise, which eventually proved near fatal as citizens became greatly outnumbered by non-citizens, and even more dangerously by helots.