CASTLE (= Computer ASsisted Tutoring and Learning Environment), our prototype for the autodidactic acquisition of the English language, combines several modern NLP (natural language processing) technologies in one intelligent system. The basic idea behind CASTLE is to bring a written medium, i.e. one or more books (such as an exercise book for students, the teacher's manual, the relating grammar, mono- and/or bi-lingual dictionaries) to an electronic platform. The appealing interface includes graphic features, such as scanned photos and illustrations as bitmaps, video sequences with hot-spots to click on, and sound in exercises of the type users might be familiar with from older language lab applications or exercises, i.e. repeating speech or interacting with the system in a speech-speech or writing-speech dialogue. Using hypertext technology, not only a clear structure of the entire system is given, but also an intelligent, fast, and efficient combination of different sources, e.g. an exercise book and a grammar tutorial, is implemented which the user can use according to his/her individual demands. The different units of a book explain different grammatical structures which the user can exercise, for example, by filling in the answers in the appropriate boxes. In the background, the answer of the user is checked by a spell- and grammar checker. According to a correct answer or type of error, the user gets the appropriate feedback immediately - with different options of how to procede, i.e. the system does not follow a rigid instruction path but rather leaves a lot of options and choices to the user. The overall modular set-up of the entire system allows an easy exchange of single modules of the system, i.e. this flexibility offers the possibility to use the "body" of CASTLE for related applications as well, e.g. CALL systems for other languages. CASTLE is presently running on any PC under Microsoft Windows 3.1. A demonstration will give insights into CASTLE's functionality and will give the opportunity to get hands-on experience with this intelligent CALL system.