…that the meter wide fortifications built at the end of the peninsula where the three rivers join together, were built to ward off danger of invasion from the Turkish Army?

In the year 1526 the Turks invaded Hungary. Panic broke out among the Lords of the castles and fortresses along the Danube. The Emperor in Vienna ordered the strengthening of the fortifications!
You can see the result of this in Passau where the efforts of 1531 a.d. ended in the widening of the walls and placing strategic holes from where the defenders could fire at the invaders entering the city from the rivers.
Of course, they never discovered if their ideas worked as the Turks never made it to Passau!