Build your library
We're not going to give a course in architectural concepts here, but we'll give the details of how the SmartEiffel utilities can help you write high quality libraries.
The simplest case is the case of a "100% Eiffel" library. In this case what SmartEiffel provides for you is its compiler and its standard library, which already contain a rich variety of features.
SmartEiffel also provides a documentation utility: eiffeldoc.
In certain cases, a library must be able to interface with the underlying system, to get access to low level functions. One can imagine, for example, creating an audio library (sorry, these days one says "multimedia"), to make system calls to access the sound card...
SmartEiffel now provides a powerful utility: plugins. You can also, even though it has been superseded, use an older mechanism, externals.