Ferdinand I, Holy Roman Emperor
Ferdinand I (Spanish: "Fernando I" ) (10 March 1503 – 25 July 1564) was Holy Roman Emperor from 1558, king of Bohemia and Hungary from 1526, and king of Croatia from 1527 until his death.
 Before his accession, he ruled the Austrian hereditary lands of the Habsburgs in the name of his elder brother, Charles V, Holy Roman Emperor.
 Also, he often served as Charles' representative in Germany and developed useful relationships with German princes.
List of monarchs of Prussia
The monarchs of Prussia were members of the House of Hohenzollern who were the hereditary rulers of the former German state of Prussia from its founding in 1525 as the Duchy of Prussia.
 The Duchy had evolved out of the Teutonic Order, a Roman Catholic crusader state and theocracy located along the eastern coast of the Baltic Sea.
 The Teutonic Knights were under the leadership of a Grand Master, the last of whom, Albert, converted to Protestantism and secularized the lands, which then became the Duchy of Prussia.
 The Duchy was initially a vassal of the Kingdom of Poland, as a result of the terms of the Prussian Homage whereby Albert was granted the Duchy as part of the terms of peace following the Prussian War.
 When the main line of Prussian Hohenzollerns died out in 1618, the Duchy passed to a different branch of the family, who also reigned as Electors of Brandenburg in the Holy Roman Empire.
 While still nominally two different territories, Prussia under the suzerainty of Poland and Brandenburg under the suzerainty of the Holy Roman Empire, the two states are known together historiographically as Brandenburg-Prussia.
 Following the Second Northern War, a series of treaties freed the Duchy of Prussia from any vassalage to any other state, making it a fully sovereign Duchy in its own right.
 This complex situation (where the Hohenzollern ruler of the independent Duchy of Prussia was also a subject of the Holy Roman Emperor as Elector of Brandenburg) laid the eventual groundwork for the establishment of the Kingdom of Prussia in 1701.
 For diplomatic reasons, the rulers of the state were known as the King in Prussia from 1701 to 1772; largely because they still owed fealty to the Emperor as Electors of Brandenburg, the "King in Prussia" title (as opposed to "King of Prussia") avoided offending the Emperor.
 As the Prussian state grew through several wars and diplomatic moves throughout the 18th century, it became apparent that Prussia had become a Great Power that did not need to submit meekly to the Holy Roman Empire.
 By 1772, the pretense was dropped, and the style "King of Prussia" was adopted.
 Thus it remained until 1871, when in the aftermath of the Franco-Prussian War, the King of Prussia Wilhelm I was crowned German Emperor.
 From that point forward, though the Kingdom of Prussia retained its status as a constituent state of the German Empire, all remaining Kings of Prussia also served as German Emperor, and that title took precedence.
Giovanni Battista Buonamente
Giovanni Battista Buonamente (ca. 1595 – 1642) was an Italian composer and violinist in the early Baroque era.
 He served the Gonzagas in Mantua until about 1622, and from about 1626 to 1630 served the Holy Roman Emperor Ferdinand II, Holy Roman Emperor in Vienna.
 Notably, in 1627 he played for the coronation festivities in Prague of Ferdinand III, son of the emperor.
 He then served as the violinist of Madonna della Steccata church in Parma.
 After a short service there, he arrived at his final position in 1633 of "maestro di cappella" at Assisi.
Karl Anselm, 4th Prince of Thurn and Taxis
Karl Anselm, 4th Prince of Thurn and Taxis, full German name: "Karl Anselm Fürst von Thurn und Taxis" (2 June 1733, Frankfurt am Main, Free Imperial City of Frankfurt, Holy Roman Empire – 13 November 1805, Winzer bei Regensburg, Electorate of Bavaria, Holy Roman Empire) was the fourth Prince of Thurn and Taxis, Postmaster General of the Imperial Reichspost, and Head of the Princely House of Thurn and Taxis from 17 March 1773 until his death on 13 November 1805.
 Karl Anselm served as "Prinzipalkommissar" at the Perpetual Imperial Diet in Regensburg for Joseph II, Holy Roman Emperor and Francis II, Holy Roman Emperor from 1773 to 1797.
List of Holy Roman Empresses
Holy Roman Empress or Empress of the Holy Roman Empire is the title given to the consort (wife) or regent of the Holy Roman Emperor.
 The elective dignity of Holy Roman Emperor was restricted to males only, therefore there was never a Holy Roman Empress regnant, though women such as Theophanu or Maria Theresa of Austria, who controlled the power of rule, served as de facto Empresses regnant.
Gerasim Zelić
Gerasim Zelić (Serbian: Герасим Зелић ; 1752–1828) was a renowned Serbian Orthodox Church archimandrite, traveller and writer (a contemporary and compatriot of Dositej Obradović).
 His chief work is "Žitije" (Lives), in three volumes.
 They are memoirs of his travels throughout western Europe, Russia and Asia Minor from the latter half of the 18th century to the first decade of the 19th century and the famous personalities (Napoleon, Prince Eugène, Viceroy of Naples, Joseph II, Holy Roman Emperor, Leopold II, Holy Roman Emperor, Francis II, Holy Roman Emperor, Semyon Zorich, Catherine the Great, Alexander I of Russia, Stanisław August Poniatowski, Dositej Obradović) he met.
 He had valuable original notes on people, religions, manners, customs, trade, etc.
Abramino dall'Arpa
Abramino dall'Arpa (fl ca. 1577–1593) was an Italian harpist and the likely grandson of Abramo dall'Arpa.
 He was one of the few Jewish musicians in Mantua in the late 16th century.
 In 1587, he accompanied and comforted the dying Guglielmo I Gonzaga on a trip to Goito.
Concordat of Worms
The Concordat of Worms (Latin: "Concordatum Wormatiense" ), sometimes called the Pactum Calixtinum by papal historians, was an agreement between Pope Calixtus II and Holy Roman Emperor Henry V on September 23, 1122, near the city of Worms.
 It brought to an end the first phase of the power struggle between the Papacy and the Holy Roman Emperors and has been interpreted as containing within itself the germ of nation-based sovereignty that would one day be confirmed in the Treaty of Westphalia (1648).
 In part this was an unforeseen result of strategic maneuvering between the Church and the European sovereigns over political control within their domains.
 The King was recognised as having the right to invest bishops with secular authority ("by the lance") in the territories they governed, but not with sacred authority ("by ring and staff").
 The result was that bishops owed allegiance in worldly matters both to the pope and to the king, for they were obliged to affirm the right of the sovereign to call upon them for military support, under his oath of fealty.
 Previous Holy Roman Emperors had thought it their right, granted by God, to name Church officials within their territories (such as bishops) and to confirm the Papal election (and, at times of extraordinary urgency, actually name popes).
 In fact, the Emperors had been heavily relying on bishops for their secular administration, as they were not hereditary or quasi-hereditary nobility with family interests.
 A more immediate result of the Investiture struggle identified a proprietary right that adhered to sovereign territory, recognising the right of kings to income from the territory of a vacant diocese and a basis for justifiable taxation.
 These rights lay outside feudalism, which defined authority in a hierarchy of personal relations, with only a loose relation to territory.
 The pope emerged as a figure above and out of the direct control of the Holy Roman Emperor.
Battle of Vienna
The Battle of Vienna (German: "Schlacht am Kahlen Berge" or "Kahlenberg"; Polish: "bitwa pod Wiedniem" or "odsiecz wiedeńska" (The Relief of Vienna); Modern Turkish: "İkinci Viyana Kuşatması", Ottoman Turkish: "Beç Ḳalʿası Muḥāṣarası") took place at Kahlenberg Mountain near Vienna on 1683 after the imperial city had been besieged by the Ottoman Empire for two months.
 The battle was fought by the Habsburg Monarchy, the Polish–Lithuanian Commonwealth and the Holy Roman Empire, under the command of King John III Sobieski against the Ottomans and their vassal and tributary states.
 The battle marked the first time the Commonwealth and the Holy Roman Empire had cooperated militarily against the Ottomans, and it is often seen as a turning point in history, after which "the Ottoman Turks ceased to be a menace to the Christian world".
 In the ensuing war that lasted until 1699, the Ottomans lost almost all of Hungary to the Holy Roman Emperor Leopold I.

Question: Italian harpist, Abramo dall'Arpa, went to Vienna at the request of the Holy Roman Emperor, who served as what?
Answer: Charles' representative in Germany and developed useful relationships with German princes