settlement layer

A settlement layer is the base chain on which a rollup's proofs are verified and its disputes are finally resolved — the supreme court of a modular stack. In the modular view, a blockchain's jobs split into execution (running transactions), data availability (publishing the data), consensus (ordering), and settlement (finalizing results and resolving disputes). The settlement layer is the last of these: the place where a rollup anchors its state, where withdrawals are honoured, and whose judgment is treated as final. For most Ethereum rollups, Ethereum itself is the settlement layer.

Concretely, settlement is performed by the rollup's contracts on the base chain. For an optimistic rollup, the settlement layer is where state roots are posted and where a fraud proof's final single-step dispute is adjudicated; for a ZK rollup, it is where the verifier contract checks each validity proof and accepts the new state root. It is also where the canonical bridge lives, so an asset's withdrawal from L2 to L1 is only truly final once the settlement layer has accepted the relevant proof. This is what gives a rollup security 'inherited' from its base chain: the heavy enforcement happens on a chain too expensive to attack.

Settlement is deliberately separable from the other roles, which is the whole point of modularity. A rollup can execute transactions itself, post its data to a separate DA layer, and still settle on Ethereum — or, as a sovereign rollup, skip an external settlement layer entirely and let its own nodes settle. Some designs even add intermediate settlement layers (a rollup settling onto another rollup, an 'L3'), each verifying the proofs of the layer above and ultimately rooting in a base chain whose finality and security everything else borrows.