Watchtowers

Basics

• The Lightning Network uses something called Watchtowers. From the Bitcoin Optech Newsletter #76 (11-12-2019):

"Watchtowers are services that broadcast a pre-programmed transaction if they detect that one of their client’s channels is being closed using an older state; this allows their clients to go offline without risking a loss of funds."

• Also from Bitcoin Optech (as of 12-2019):

"Watchtowers send LN breach remedy transactions (justice transactions) when they detect that one of their client’s counterparty has broadcast an outdated channel close transaction.

The service provided by watchtowers allows their clients to go offline for significant amounts of time without having to worry about their funds being stolen by a counterparty. Watchtowers are not entrusted with any funds, just the responsibility of monitoring the blockchain and broadcasting transactions, although breach remedy transactions can be designed so that the watchtower receives a portion of the safeguarded funds if their services are needed."