Data Fortress (Datafort for short) is simply a computer system. It is a 3D representation of that computer within the Net. The specific form the fortress takes depends on the the system it is hosted in...as with icons, more realistic environments will require more resources (memory). The default rendering for data fortresses looks like something similar to Tron. But they can be as elaborate and photo-realistically detailed as The Matrix. You could make your data fortress look like a castle, or a cruise ship or a space station. Within the data fortress, the sysop (System Operator...the person in control of the Data Fort) determines what it looks like. The only limitations are the system's own resources.
Data Fortresses have data walls. These represent how hard it is to enter the system. Data wall strength will depend on the amount and quality of resources of the system. There are programs that can penetrate data walls and allow a Netrunner to move through them. All Data Fortresses have code gates (which will be rendered as actual doors, or something similar), which are the normal way of getting inside a system's data walls.
Within the Datafortress, various parts of the system will be rendered in intuitive ways. For example, files may be stored in a locked office (a section of memory in the system) in what looks like an actual filing cabinet. So your icon would open the filing cabinet and take out the file...giving you access to the information.