This repository implements the 'classic' Arbitrum stack, an optimistic rollup L2 scaling solution for Ethereum. It is a multi-language monorepo containing a custom C++ Arbitrum Virtual Machine (AVM) for deterministic execution, a Go-based node (sequencer, validator, RPC server, inbox reader), and Solidity smart contracts for L1-L2 bridging (Inbox, Outbox, Rollup, Challenge, SequencerInbox) and token gateways. The system processes L1 transactions via the SequencerInbox, executes them off-chain on the AVM, posts state commitments to L1, and serves Ethereum-compatible RPC endpoints. Validators can challenge fraudulent assertions through interactive fraud proofs. The codebase is well-structured with clean abstractions for caching, broadcasting, and transaction authentication, but it is explicitly superseded by the Nitro upgrade and no longer actively developed. It demonstrates solid engineering practices for a complex blockchain infrastructure project.