ActivityTimeline Cloud is hosted on Amazon AWS with Amazon Aurora as a data storage. The information is stored in the data centre physically located in Frankfurt, Germany or in Ohio, USA. The data centre is secured and under constant monitoring with industry level Amazon's security.
By default, ActivityTimeline Server stores all of its cached data in a data file on the server where ActivityTimeline Server is installed (it is located at /path/to/activitytimeline/bin/atdb folder). This data file can be accessed by your system administrator who has an access to the file system on the server only. Also, ActivityTimeline can be reconfigured to use an external database like MSSQL, MySQL, Oracle and PostgreSQL to securely store its data. Here is guide on how to do it: https://activitytimeline.atlassian.net/wiki/spaces/AC/pages/46307061