Roswell keeps its effect on the system to the minimum.
Installing roswell on linux causes the core C runtime (ros binary) in /usr/local/bin/ and a few lisp files under /usr/local/etc/roswell. They are completely safe to remove once you decide to remove roswell.
You can optionally change this installation directory by ./configure --prefix=/path/. See Local installation.
Roswell DOES NOT use this directory for its runtime operation (subcommands like ros install sbcl).
Files downloaded/created/manipulated by Roswell are kept under ~/.roswell directory. It is completely safe to remove this directory without affect the integrity of the system, nor roswell itself. Removing this directory means resetting Roswell to the blank state, just like removing ~/.mozilla for firefox which removes all of your preferences/history/plugins.
Quicklisp, ASDF, or Quicklisp libraries installed by Roswell does not affect, nor is affected by, the Quicklisp, ASDF, or the libraries that you installed manually. Roswell downloads and keeps them separately under ~/.roswell.
Implementations installed by Roswell does not affect, nor is affected by, the implementations already installed on the system. However, roswell is able to use those systems by ros use <IMPL>/system (e.g. ros use sbcl/system) or ros -L <IMPL>/system ... which uses the binary visible from PATH.
Roswell does not write to the system configuration files nor user configuration files outside roswell, such as .profile, .bashrc, .emacs or .sbclrc. Nor does Roswell magically alter the environment variables like PATH.