Imagine using mobile guards to defend the vertices of a graph G from a sequence of attacks subject to the conditions that after each attack: (i) each guard either remains in place or moves to an adjacent vertex; (ii) the configuration of guards forms a Roman-dominating set; and (iii) there is at least one guard on each attacked vertex. We show that it is always possible to defend the vertices of a tree with n vertices using at most 5n6 guards and that this bound is tight.