To the Temple
It was widely understood that the goddess Sulis Minerva had the power to punish thieves. Even if the identity of the robbers was unknown, she could find them and exact revenge. Upon arrival at the temple grounds, Solinus would need to write a curse text on a piece of lead, for which he probably needed the help of priests and maybe a professional scribe. The professionals knew exactly how to address the goddess, and how to get requests to reach her.
With his inscribed tablet, Solinus would go to the sacred spring. The spring had been enclosed in a room with a high vaulted ceiling, which was only accessible through one narrow door.