The linked data Web provides a simple and flexible way of accessing information resources in a self-descriptive format. This offers a realistic chance of perforating existing data silos. However, in order to do so, space, time and other semantic concepts need to function as dimensions for effectively exploring, querying and filtering contents. While triple stores, SPARQL endpoints, and RDF were designed for machine access, large burdens are still placed on a user to simultaneously explore and query the contents of a given endpoint according to these dimensions. First, one has to know the semantic concepts and the type of knowledge contained in an endpoint a-priori in order to query content effectively. Second, one has to be able to write and understand SPARQL and RDF. And third, one has to understand complex data type literals for space and time. In this article, we propose a way to deal with these challenges by interactive visual query construction, i.e., by letting query results feedback into both (space-time) exploration and filtering, and thus enabling exploratory querying. We propose design principles for SPEX (Spatio-temporal content explorer), a tool which helps people unfamiliar with the content of SPARQL endpoints or their syntax to explore the latter in space and time. In a preliminary user study on a repository of historical maps, we found that our feedback principles were effective, however, that successful question answering still requires improvements regarding space-time filtering, vocabulary explanation and the linking of space-time windows with other displays.