Here you find the download of the free BTL viewer. The viewer supports the modern BTLx as well as the classic BTL format.
Download BTL-Viewer Build 19.5.400 dated 18.09.25: btlviewersetup.exe
Release notes for the BTL viewer: release_btlviewer.pdf
An overview of BTL viewer and the release notes can be found here: https://github.com/design2machine/BtlViewer/releases