I build a simulation engine, based on a high performance distributed actor system library I wrote for just this purpose. It is generic, but a lot of the performance trade-offs are with Citybound's needs as the main driver.
Their actor implementation / event loop works on a higher level than mine. Mine is super simplistic intentionally to offer high performance. Being optimised for cache locality, processing similar actors in batches, using custom allocators and having encoding-free communication, it has many parallels to data-driven game engine design or even entity component systems.
