When building the project with OpenJDK 11, compilation fails with the following errors (full context omitted for brevity, there are lots of those): error: package com.sun.org.apache.xml.internal.serialize is not visible import com.sun.org.apache.xml.internal.serialize.OutputFormat; ... error: package com.sun.org.apache.xml.internal.serialize is not visible import com.sun.org.apache.xml.internal.serialize.XMLSerializer; ... error: cannot find symbol ... BASE64Encoder().encode(encodeStringtoByte(raw));...
Currently, source code *.java files still use 8-bit encoding for the second half of the ASCII table, mostly for comments in German. This upsets javac which spits lots of error: unmappable character for encoding ASCII unless run with -encoding ISO-8859-1 switch. Consider converting them all to UTF-8 which would alleviate the need to pass encoding explicitly and would generally simplify working with the source code in modern environments.
The LevelManager class loads levels as "Level_" + lvlId + ".txt" (notice the uppercase L), but version 1.0.3 ships level sets 6 and 7 named starting with lowercase l. This probably does not matter on Windows, but breaks on Unix. Renaming those files allows the game to load them.