You can search for this cache at any time but I strongly recommend to pick it up only in the very early morning or at night! Once you are at the cache location you will understand why. Note that every stage can be found at night without a pocket light. Besides the right timing you will also need a measuring tape and a city map. A compass can be useful but is not absolutely necessary.
The start location, stage 1, can be reached best by subway U1 or U3. Get off at station "Stephansdom".
Stage 1: start
N1 = 48° 12.360
E1 = 16° 22.280
You should find yourself in the middle of a great circle. Count the number of all round flat stones which are inside the circle (including the one you are probably standing on right now). This number is A.
Okay, this was not difficult, was it? So let's move on to stage 2.
N2 = N1 + (A*4*4)-4
E2 = E1 + (A*5)+5
From this stage go about 50 meters to direction north east. For some interesting reasons, there are two metal bars, about 1.2 meters above the ground, mounted in the wall.
Measure the length of the short metal bar (from outer edge to outer edge in cm). Divide the length by 10 and round it to the nearest whole digit. The final result for the short metal bar is Y. Do the same with the long metal bar. The final result for the long metal bar is C.
Not far away from your current position at the metal bars, there is a secret digit protected behind a glass plate. Let's name it D.
Measure the width of the glass plate (in cm) and again, divide it by 10 and round it to the nearest whole digit. The result is E. Now, as you have practice in doing this, you can calculate the result for the height of the glass plate in the same manner. The final result for the height is F.
N3 = N2 + (A*Y - C*D - E*F) * D
E3 = E2 + (A+Y+D) * D
Take a look at the roof. You will see 4 big digits there [=NI GS].
N4 = N3 - ((next number in sequence F E Y ?)*3+G)
E4 = E3 + (G*I*N) - (A*C) - F + S
At stage 4 you can't overlook a species which is quite rare in nature. Don't be afraid ;-) Just multiply the big numbers at the top of the species. The result is H.
Finally, take a look at your city map. The total number of stations of the subway called "U[A-E-H+I]" is T.
Now, as you are in the possession of all variables, it's time to put everything together and calculate the cache coordinates.
N5 = N4 + (T-H-E)+(C*A-C*H-E)+(I-S)+(H+I+D+D+E-N)+10
Stage 5: cache
E5 = E4 - (A*N*D)-(T-I+N+Y)-5
The GPS reception at the cache location is probably not impressive. You will get the best accuracy when you are about 10 meters away. By the way: don't let yourself be fooled by your GPS device - check the number of satellites and their position in order to determine the quality of your indicated postion.
The micro cache (a 35 mm film container) is hidden in about 2.10 meters height. Once you spotted it, take your time and wait for the right moment when really no one is watching you. Again, perfect timing is decisive for the success of your mission. Please be very stealthy and rehide the cache very carefully!