Osterstein Castle location
According Wikipedia there are two Osterstein castles one is in the town of Zwickau.  The river Zwickauer Mulde flows through it but not the Elster river.   The castle is in Gera.   The White Elster flows through Gera so that must be the proper one since the invitation says the "Castle Osterstein on the Elster". The castle was rebuilt 1686-1735 so it was only completed 11 years before the wedding.  It is still listed as a major attraction in Gera.
Until their branch of the royal line ended in 1802 the castle was the seat of the line of Reuss-Gera (Reuß-Gera in German)
A tower (the keep) is all that remains, the rest was largely destroyed in World War Il.

The brides family is Reuss  (Reuß in German), Reußen means of the lineage of Reuß.  The family is described as:
Counts and Misters von Plauen, Misters Graiz, Cnannichfileld, Gera, Schleiz and Lobenstein, etc.

Plauen 50 KM South of Gera
Graiz does not exist but Greiz does, it is 30 KM south of Gera
Cnannichfileld may actually be Kranichfeld In which case it is 80 KM West of Gera

Gera is in the state of Thuringia and is the third largest city in that state. It is also where the wedding was held. It is close to the western border of Saxony.   It is where the wedding invitations were printed and is associated with the bride’s family.   
Schleiz 50 KM south south west of Gera.
Lobenstein, Bad Lobenstein, in the state of Thuringia, is 70 KM south south west of Gera
As a side note Reußen is also a tiny town 90 KM North of Gera

There is an 80 page book  about the bride's father's family history. "Herrn Heinrich des  XXV of the house of Reuß".  The book was published in German by subscription for interested parties.  It is a chronicle of the bride's family,  the royal family of Reussen von Plauen.  This chronology begins in 1307.  Two sets of eyes could not find any mention of Ludwig.  There were several Christiane Sophie Henriettes born around the time of the wedding.  Most men in the family were called Heinrich so it is difficult to distinguish who was who. 
 
The bride's father was born 27 August 168? and died 13 March 1748. He was married twice.  His first wife died in 1718.  He remarried in August 1722 to Sophia Maria and she is the mother of the bride.

Most of the book dealt with births, deaths, exchange of land and special monetary payments.  One bit of interesting information in it was that the city Gera was destroyed by fire on 18 September 1780, 26 years after our Ludwig immigrated to America.  Both churches, city hall, jail, school and all other public buildings were destroyed.  Just a few houses remained.  If Ludwig lived in Gera as it is assumed from the invitation, all records would probably be lost. If the papers escaped that disaster, they would have also have had to survive the heavy bombing of Gera during World War II.  The book is part of the Google Books Collection.  It was published in 1811 in Leipzig, about 70 KM north north east of Gera.  1811 is just 9 years after the end of that branch of the royal line of Reuß-Gera or in English, Reuss-Gera.

The Groom, Count and Lord Frederick  Bodo 
Described as
of H.R. Empire Count of Stolberg, Königstein, Rochefort, Wernigeroda and  Hdrenstein, Lord Epstein, Munzenberg, Breuberg, Aigmont, Lohra and Clettenberg  and Thro Kömigl. Majesty in Pohlen and Churfurstl. Highness of Saxony, High appointed position of major general and colonel over a foot regiment.
Stolberg, there are 3 of them discussed in detail below.  The most likely one is 60 KM east south east of Gera and in the state of Saxony.
Königstein It is in the state of Saxony 270 KM east south east of Stolberg.
Rochefort.  The only Rocheforts I can find are in France and Belgium.
Wernigeroda does not seem to exist but Wernigerode (ending with an "e" not an "a") does exist.  It is about 250 KM north east of Stolberg in the current state of Saxony-Anhalt.   The spelling in the original manuscript is clear. The last letter is an "a".  I do not know if spelling changed or if this word refer to something else.
Hohenstein, there are 2 of them.  One is about 250 KM north east of Stolberg near the Belgium border and the other is now called Hohenstein-Ernstthal and is about 15 KM north north west of Stolberg    
Lord Epstein.  There does not seem to be a town of Epstein but it is an interesting name. The surname Epstein is one of the oldest Jewish family names in the Slavic countries such as the Czeck Republic that is about 50 KM south of Stolberg.
Munzenberg  a town about 15 KM north of Stolberg.
Breuberg 400 KM west south west of Stolberg  it is a small town near Frankfurt
Aigmont  Not a city in Germany and not a very common last name.
Lohra 370 KM west of Stolberg a little north of Frankfurt
Clettenberg is not a city in Germany but Plettenberg is.  It is 400 KM west of Stolberg, and north west of Frnakfurt.  Could the first letter be a "P" rather than a "C"?
Thro Kömigl   I can not find anything on this.
Majesty in Pohlen and Churfurstl
         Pohlen, Linda bei Weida is 60 KM west of Stolberg.
         Churfurstl is more of a puzzle.  There is no city by that name. It is another reference and title
         applied to the upper classes and aristocrats during that era. 
         I find “Chursurstl” in Google including one reference of “Pohlen und Chursurstl” from 1756
         but I can not figure it out.  I can not even find the words "Pohlen" and Chursurstl" at the reference.

Stollberg:
The groom was H.R. Empire Count of Stolberg so I am assuming he was from Stolberg.  Germany was not unified at this time so I would think you would have to live close to what you were the count of.  There are three possible locations for Stolberg according to Wikipedia.

1) on the Western border, Stolberg (Rhineland)  (500 KM west of Gera, where the bride was)

2) In about the middle of the country, Stolberg, Saxony-Anhalt (150 KM north west of Gera)

3) Just north of the western end of the Czech Republic, Stollberg which is also called Stollberg/Erzgeb and Stollberg/Erzgebirge (60 KM east south east of Gera)  in the Free State of Saxony now just called Saxony.  It is the southern most state in what used to be East Germany.  These are connected towns or perhaps one town with about 12,000 inhabitants, 3,000 in 1834.  The spelling of Stolberg is different, one "l" in the wedding papers and two on the map but considering how lax people used to be with spelling I do not see that as a problem since this is substantially closer than the other 2, I will assume it is the correct one.


The Elster River
Elster a river that the invitation says the castle is on. There is the black (Schwarze Elster) and the White (Weisse Elster)  The city of Zwickau is not on either one although they both are in the same general area.  The river that runs through Zwickau is the Zwickauer Mulde not the Elster.    The White Elster is about 20 KM west of Zwickau and runs through Gera, the town where the invitations were printed and the town associated with the bride’s family.

The Black Elster is about 160 KM east north East of Gera so it is not likely to be the one referred to.
The White Elster or Weisse Elster (German: Weiße Elster ) goes through  Plauen, Greiz, Gera, Zeitz, Pegau and Leipzig. It flows into the river Saale in Halle.

The Black Elstter, or Schwarze Elster, flows through the cities Kamenz, Hoyerswerda, Senftenberg, Lauchhammer, Elsterwerda, Bad Liebenwerda, Herzberg and Jessen. It flows into the river Elbe at Elster (Elbe), upstream from Wittenberg.


Unanswered Questions

1) One would assume that a printer who got his name on the wedding announcement would be fairly old and important enough to not be a likely candidate to immigrate.
2) Was there any obvious political turmoil near Gera is in the state of Thuringia around 1754 when Ludwig immigrated.
3) It is relatively well established that Ludwig died in 1792, 38 years after he immigrated to America in 1754. This would have been 46 years after the wedding 1746.  It is odd that someone who was important enough to get his name on the invitation would live another 46 years.  
4) One would think that if our Ludwig was an accomplished printer, he would have worked as one in this country.  It is possible that lack of skill with English forced him into another line of work.  
5) It would be useful to see if there is any connection between our Ludwig and printing in the area he lived, York Co., Pennsylvania possibly in Dover Township which since the early 1800s  has been known Conewago Township or Newberry Township or York (town) in York county.
5) It is possible that the two Freysingers are not brothers, one might be a father or uncle.
6) Can any other data on Christian Willhelm Freysinger be found in Germany.
7) In the list for the groom's family connections there are a number of puzzles.