Here the Lahn river flows into the Rhine (Ren). Lahnstein consists of the districts Oberlahnstein (above the river mouth) and Niederlahnstein (beneath the river mouth).

Because of its strategic importance at the Rhine river and the hard-fought street of nations, Lahnstein was heavily fortified. Many old gates and towers still now demonstrate its importance in the Middle Ages. Lahneck Castle thrones high above the city of Lahnstein. It was built in 1240-1245 and served as a domicile of the archbishops and electors from Mainz. Also well worth seeing is Martinsburg Castle on the bank of the Rhine in Oberlahnstein.

Lahnstein is a perfect starting point for sportive activities both in the Rhine and the Lahn Valley. Special highlights are the walking-trails Rheinsteig and Rhein Burgen Wanderweg and also the romantic castle and palace bicycle tour and canoe tour.