Walsingham was Elizabeth's SOS from 1573, who had a 'spy network' where for example he had members of the government, e.g. LLs and bishops to forward potential threats to him. Furthermore, in 1580, he had spies operating in key locations such as France, Germany, Spain, Italy and even Istanbul. He also made us of double agents to infiltrate Catholic Plots
Acts of Parliament led by him increased severity of punishments for Catholics to include executions. 130 priests were executed during Elizabeth's rule
Captured Catholics faced torture for information on plots. Though he didn't like the use of torture, he considered it necessary for Elizabeth's safety.
Walsingham used ciphers to write secret messages and codebreakers to decode plots. He employed a man who could re-seal the plotters' letters. Criminals were also given the option in order to avoid punishment to become spies and be apart of Walsingham's Spy Network