Some of the strongholds that have thus been restored are now great fortresses, held by the governors of the states and kingdoms that border on the river; others of them are fitted up as summer residences for the persons, whether princes or private people, that happen to own them. About midway between the beginning and the end of the mountainous region of the Rhine is a place where there are two very important works of this kind. One of them is far the largest and most important of all on the river. This is the Castle of Ehrenbreitstein. Ehrenbreitstein is not only a very strong and important fortification, but it guards a very important point. This point is the place where the River Moselle, one of the principal branches of the Rhine, comes in. The valley of the Moselle is a very rich and fertile one, and in proportion to its extent is almost as valuable as that of the Rhine. The junction of the two rivers is the place for defending both of these valleys, and has consequently, in all ages of the world, been a very important post. The Romans built a town here, in the days of Julius Caesar, and the town has continued to the present day. It is called Coblenz. The Romans named it originally _Confluentes_, which means the _confluence_; and this name, in the course of ages, has gradually become changed to Coblenz. Coblenz is built on a three-cornered piece of flat land, exactly on the point where the two rivers come together. There is a bridge over the mouth of the Moselle where it comes into the Rhine, and another over the Rhine itself. The bridge over the Moselle is of stone, and was built a great many hundred years ago. That over the Rhine is what is called a bridge of boats. A row of large and solid boats is anchored in the river, side by side, with their heads up the stream, and then the bridge is made by a platform which extends across from boat to boat, across the whole breadth of the stream. Near the Coblenz side of the bridge there are two or three lengths of it which can be taken out when necessary, in order to let the steamers, or rafts, or tow boats, that may be coming up or down the river, pass through. Rollo was very much interested, while he remained at Coblenz, in looking out from the windows of his hotel, which faced the river, and seeing them open this bridge, to let the steamers and vessels pass through. A length of the bridge, consisting sometimes of _two_ boats with the platform over it, and sometimes of _three_, would separate from the others, and float down the stream until it cleared itself from the rest of the bridge, and then would move by some mysterious means to one side, and so make an opening. Then, when the steamer, or whatever else it was, had passed through, the detached portion of the bridge would come back again slowly and carefully to its place.