Complex sentence: It is situated at the village of Upnor, opposite and a short distance downriver from the Chatham Dockyard, at the time a key naval facility, and was intended to protect both the dockyard and ships of the Royal Navy anchored in the Medway.
Split into simpler sentences: It is situated at the village of Upnor, opposite and a short distance downriver from the Chatham Dockyard, at one time a key naval facility. The fort was intended to protect both the dockyard and ships of the Royal Navy anchored in the Medway.