There is a potentially endless variety of fortresses. Historically I have focused on five main types. At some recent workshops I have conducted, participants have reached a consensus that one more type would be useful, so now we are officially up to six. Some organizations may find it convenient to add types I have not considered. So don't consider this a final list, but rather a collection of software fortress patterns.
In any case, here is my current list of the six “standard” fortress types, along with the responsibilities of each and the abbreviations I use to refer to them throughout the book. I will give only an overview here: