This monorepo implements the classic Arbitrum optimistic rollup Layer-2 solution. The system consists of a custom Arbitrum Virtual Machine (AVM) written in C++, a Go-based node that provides an Ethereum-compatible RPC interface, sequencer/validator functionality, and Solidity smart contracts on L1 for bridge, rollup, challenge, and staking logic. Key data flows: the inbox reader ingests sequencer batches and delayed messages from L1, delivers them to the AVM for execution, and produces state updates and transaction results that are served via the RPC API. The architecture supports multiple node modes (forwarder, aggregator, sequencer, validator) with Redis-based lockout for sequencer exclusivity and optional Fireblocks integration for external signing. Despite being classified as 'classic' and superseded by the Nitro upgrade, the codebase continues to receive active maintenance and contains production-grade infrastructure for running Arbitrum nodes.