The Romans loved to build things out of stone, which is why so many of their structures have survived to become tourist attractions. The Germanic tribes, on the other hand, were much more fond of wood construction. The Germans soon came to appreciate the Roman approach, although building with stone was much more expensive. The considerable Roman engineering and architectural skills were sorely missed, however, as most of these were lost during the Dark Ages and had to be re-invented. Meanwhile all sorts of sturdy Roman structures --fortresses, arenas, theaters, even tombs-- were pressed into service as strongholds, some continuing in such service well into the twentieth century. For example, the fallout shelter for the President of France is built inside the foundations of a ancient Roman fortification.