Detinets is a technology in Age of Empires II: Definitive Edition - Dynasties of India that is unique to the Slavs and can be researched at the Castle. Once researched, it replaces 40% of Castles' and towers' stone cost with wood. Castles cost 260 wood, 390 stone after researching this technology, and towers cost 100 wood, 75 stone.
Note that it also adjusts the repair cost of Castles and towers.
Stone is the most precious resource, as it can only be obtained by mining or Market purchase. In a very similar manner to the Franks' bonus, this means that the initial, mineable supply of stone is able to erect several more buildings before forcing the player to buy stone on the Market. One thing worth considering is that, besides the obvious defensive potential, more Castles make it easier to mass Boyars.
Unlike the Franks' bonus, however, the player has to build a Castle to even research this, so the first Castle cannot benefit from this bonus (except for rare maps/settings that give the player a free starting Castle).
It also affects towers, which presents an interesting option for the Slavs — their towers lack several important upgrades, but most of these lie in the Imperial Age (meaning they're as strong as other civilization's towers during the Castle Age), and even in the Imperial Age, the size of the stone discount may be able to justify the lower power potential, giving their towers some qualities of a "trash unit".
It's important to note that the vaguely worded "Military Buildings provide +5 housing" for the Slavic team bonus does not apply to either towers or Castles.
Detinets (Russian: Детинец (Detínec), lit. "Citadel") is an ancient Rus' city-fort or central fortified part of a city, similar to the meaning of kremlin (fortification), citadel. The term was used in the Kievan Rus', in Chernihiv, Novgorod the Great, Kyiv, and others.