Our provider has investigated the issue that occurred and has provided a full report. The incident report can be viewed on our provider's status page.
The overall cause of the increased 504 errors was a DDoS flood attack that managed to bypass existing firewall and protection rules. As this traffic was not blocked at the WAF (web application firewall) level, it disrupted Craft Cloud applications, allowing DDoS flood traffic to pass through the firewall.
Craft Cloud has taken steps to increase resources in the Europe region to help mitigate future issues related to DDoS scenario. Further work is being carried out in the long term to improve resiliency and minimise further disruptions, including improving of detection if legitimate traffic as well as bot/DDoS traffic.
For requests that had been cached through the full-page static caching layer, these requests would have continued to be served without issues. Any request that is dynamic or not cached would have been impacted, given it would be impacted by increased latency due to the DDoS.