There are two layers

"A quick note about how Istaria's server works - there are two layers, a core framework written in C++ (what we call “the server”) and game logic (what we call “the simulation”)."
https://community.istaria.com/news/?id=961

Are both written in C++?

"Engine
Intrinsic Alchemy
Mantrid Engine"

https://en.wikipedia.org/wiki/Istari..._of_the_Gifted

Can you explain these two Engine in short?

Hi there! On the server we have the core framework which is written in C++, and then the gameplay code ("the simulation") which is written in a bytecode based language which is generated from another language called Xev which is similar to Lua / Python (so you could say the server gameplay code is written in scripts).

Intrinsic Alchemy is the engine the game client uses, whereas Mantrid is the name of the server core framework, so the game you have on your computer uses Alchemy but the game's server uses Mantrid. Hope this cleared it up!