Nestled in the hill 300 feet above the city of Heidelberg stands the breath-taking Heidelberg Schloss (castle). The castle is a combination of several buildings surrounding an inner courtyard, put together with a haphazard look. Each building highlights a different period of German architecture. The castle ruins are among the most important Renaissance structures north of the Alps. The castle has only been partially rebuilt since its demolition in the 17th and 18th centuries.

The castle has a history almost as old as the city itself. The earliest castle structure was built before 1214 and later expanded into two castles circa 1294 but in 1303, lightning struck the upper castle and destroyed it.  It wasn't before Prince Elector Ruprecht III (1398 - 1410) that the castle was used as a regal residence. When Ruprecht became the King of Germany in 1401, the castle was so small that on his return from his coronation, he had to camp out in the Augustinians' monastery, on the site of today's University Square. What he desired was more space for his entourage and court and to impress his guests, but also additional defenses to turn the castle into a fortress.  Every King resident afterwards continued it's expansion which accounts for the varying architectural styles. 

The country of Germany was mostly divided between several royal lines, and Heidelberg castle became not only royal house but also important military stronghold that was involved in many skirmishes. The first time the castle was attacked was during the destructive Thirty Years' War (1618-1648) in which European Protestant States fought against Roman Catholic States and caused death of over 8 million people. During this war, Frederick V left castle undefended, which enabled General Tilly (commander of the Catholic League's forces) to capture the castle defenses in 1622. After that siege, the castle was involved in several more skirmishes before the end of the war.

Castle Heidelberg was also a part of the Nine Years' War between France and the Grand Alliance, in which Germany was a part of the Holy Roman Empire. During that war, French took hold of the deserted Heidelberg castle in 1688, and retreated from it in 1689 while setting it on fire and destroying its fortifications and defensive structures. In the decades after this war, the castle remained in ruin and only the most basic renovations were performed.

Until it was hit once again and destroyed by lightning in 1764 leaving it permanently uninhabitable, the castle was the residence for most of the Prince Electors. In 1800, Count Charles de Graimberg began the difficult task of conserving the castle ruins. Up until this time, the citizens of Heidelberg had used the castle stones, irons, and decorations to build new their own new houses.

Salvation of the castle came not from nobles, royals, and German politicians, but from artists, who started painting and promoting the castle across the entirety of Europe, giving the castle vocal support from several European lords, most notably French count Charles de Graimberg. In the second part of the 19th century, German poets and historians started arguing for expensive and complete restoration, which finally happened between 1897 and 1900. During all that time, the castle became more and more famous, and tourists from Europe and North America started visiting it in ever increasing numbers.  Today, in the 20th century, Americans spread Heidelberg's reputation outside Europe. Thus, Japanese also often visit the Heidelberg Castle during their trips to Europe. Heidelberg has, at the beginning of the 21st century, more than three million visitors a year and about 1,000,000 overnight stays. Most of the foreign visitors come either from the USA or Japan

Just as breath-taking as the castle is from the city, so too is the city from the castle. From either the Great Terrace or the gardens, one has an amazing view of Heidelberg, the Neckar River, and the Neckar valley far into the Rhine plain. On a clear day, Mannheim is even visible on the horizon.

The castle gardens, constructed between 1616 and 1619 by garden architect Salomon de Caus, were commissioned by Prince Elector Friedrich V for his wife Elizabeth. Before being destroyed during the War of the Palatine Succession in 1693, the gardens were regarded as a masterpiece of their time. The gardens, built upon several terraces, were made up of many flowers beds, mazes and arbours, numerous sculptures, a heated greenhouse with orange trees, large fish ponds, waterfalls, and a man-made grotto for musical water arts.

There are three times every summer when the well-known Schlossbeleuchtung and fireworks display takes place at the castle ruins– the first Saturday’s in June and September, and the second Saturday in July.  The castle lighting is done in memory of the three times when the castle went up in flames (1689, 1693 and 1764). The first two times were due to wars with the French, and the last time by lightning.  After the castle 'burning' begins, a 15 minute fireworks display is launched from the Old Bridge.