Definition
A watchtower is a service that monitors the Bitcoin blockchain on a Lightning node's behalf, defending its channels against breaches even while the node is offline. Lightning's security model assumes participants stay online to catch cheating, but real-world nodes go offline. Watchtowers close that gap.
The breach problem
Because every past commitment transaction remains technically valid, a dishonest peer could broadcast an old, more favorable channel state while you are offline, attempting to steal funds. The protocol's defense is the penalty (or justice) transaction, which lets the wronged party sweep the entire channel balance using the cheater's revocation secret. But that defense only works if someone is watching when the breach hits the chain.
How a watchtower defends you
You hand the watchtower encrypted, succinct hints derived from each channel update. The watchtower stores these blindly. If it ever spots a revoked commitment confirming on-chain, it uses the matching hint to decrypt and broadcast the pre-signed penalty transaction, reclaiming your funds, often routing the swept balance to an address you control. Critically, a well-designed watchtower never learns your channel balances or payment activity.
Watchtowers protect the revocation guarantees built into every payment channel and are especially valuable for always-on routing nodes.
In Simple Terms
A watchtower is a service that monitors the Bitcoin blockchain on a Lightning node’s behalf, defending its channels against breaches even while the node is…