Fantastic Beasts

If you do all informative stages on foot on one occasion it is a pretty long walk, what accounts for the 2 stars in terrain-rating. If you prepare for the walk you may decide to start at Stage 3 and do Stage 1 and 2 afterwards (as I did when collecting coordinates), but I had to change the order for "dramaturgical reasons" as you will see (or already guess while reading).
The walk can of course be done any time of day, but the places will be less crowded in the evening after all shops have closed. As most of the main attractions are illuminated anyhow I suggest the evening for finding the magic word and daytime for logging the cache (if you go to the cache by night a torch might be useful). If you have children with you I strongly suggest NOT to go there in the dark (especially not during the warmer season).
Except the last 4 meters everything is paved and basically flat (apart from kerbstones, maybe a few steps in the area of Stage 11 and some sort of ramp at one place).

At each stage you will have to find a letter. I've assigned indices to the letters (a, b, c, ...), the magic word will have the form abcd... .
Different methods of deriving letters will be used (apart from using found letters directly): Conversion of numbers (1=a, 2=b, ... 26=z) and finding the counterpart using the decrypting table (to be found on any cache-printout).

Some of the information may be found by googling, but I hope that at least some of the letters can be found only "in situ". The GPS-signal will be pretty bad most of the time, therefore I've put all coordinates into the description. Thus you can prepare the stages on a city map. At some places I've put a little help (only a little, this is supposed to be a challenge into the hints. As long as the coordinates I've taken made sense to me when plotted on the map I didn't change them.

Stage 1: "Norbert"
N 48° 12.635', E 016° 22.158'
You are standing in front of a house more than 300 years old, little "Norbert" is sitting above the entrance. Look around for an object that makes absolutely no sense at the place where it is attached to the house wall. There are some letters on it, the third letter will have the index d.

Stage 2: Chimera
N 48° 12.431', E 016° 21.940'
Look to the left (if you are coming from Norbert's place): There is a group of strange creatures. On the other side of the yard, on top of the tower, some sort of "horse-fish" is facing the wind.
Add the number of creatures to the horsefish, convert the result to a letter and find the decryptic counterpart (index q).
You can shorten your way to Stage 3 a bit by using some public transport along the Ring.

Stage 3: The Owlery
N 48° 11.972', E 016° 22.096'
Many birds guard this house of wisdom. Count the smaller ones, convert their number to a letter (index b).
Don't walk round the house (unless you wish to), you'll need only the owls you can see from where you should be standing.

Stage 4: The Inventor
N 48° 11.960', E 016° 22.175'
It is due to the work of such people that muggles can get along without magic so well. Take the middle letter of the english word for his most famous invention (index l).
(I refer to the invention that is indicated on the object, although there may be doubts about the historical proof for it.)

HINT: During the Advent season there is a charming little market in this area.

Stage 5: The Serpent
N 48° 11.911', E 016° 22.300'
A somewhat strange team is standing guard here. The snake seems to be trying to tell us something, but what? About 20 meters to the right of the serpent you will find an info-table which bears a number on the lower right side. Sum up the digits of this number and convert the result to a letter (index c).

Stage 6: The Sign of the Griffin
N 48° 12.271', E 016° 22.277'
You have just entered one of the not so busy roads in Vienna. Find the griffin and look for the number to the right of it. Convert the first letter of the lane's name to a number and add it to the number to the right. Convert the result to a letter and find its counterpart (index k).

Stage 7: Heraldic
N 48° 12.280', E 016° 22.459' (this one was pretty inaccurate, so I've altered it)
This is a quite common species here in town (some of them much larger, many of them much smaller), but very rarely to be seen in real life. Substract the number of its heads from the number where you found it. Convert the result to a letter (index o).
Look out for some famous entertainment buildings while you are walking down this street.

Stage 8: The Magic Fountain
As Pigwidgeon has disappeared the lovely Mermaids have taken over the job to give you a hint. You will find them at N 48° 12.652', E 016° 21.917'. Winters are cold here, so the Mermaids reside inside, of course, but there is a convenient passage to visit them and listen to their song: "Answer our question and we will tell you the letter you are looking for! What does the human on top of the fountain hold in its hand?"
paddle = D
fish = E
net = F
Take the correct answer for index f.

Stage 9: "Gringotts"
N 48° 12.332', E 016° 22.393'
An ancient greek hero (holding up the head of his enemy) is guarding the entrance to a formidable residence. Once a well renowned prince used to live here, later a much less liked company took over the place. Note down the last letter of the surname of the famous architect (index i).

in case Gringotts is invisible you can make a detour via Weihburggasse to Stage 9a

Stage 9a: "Giants"
N 48° 12.358', E 016° 22.485'
Two giants are holding up the walls of an old house since the year ????. Convert the second digit into a letter and use it as index i.

Stage 10: A touch of Knockturn Alley
N 48° 12.436', E 016° 22.520'
Stage 10 and 11 will take you through a set of medieval houses and yards. If you come here during daytime you will see more of the architecture, but after nightfall this area becomes quite spooky.
The name of the lane where you should be standing right now has the german word for a colour in it. Note down the last letter of the german word for its complementary colour (index h).

NOTE: Before entering the place through the black iron gate you may have a look at Nr.8, where a very important breakfast-item was invented.

Stage 11: Hidden Treasure
N 48° 12.448', E 016° 22.462' (coordinates don't seem too wrong)
Deep below the roots of this tree there lies the treasure of the templars who have been massacred 7 centuries ago (according to legend their blood has flown down a nearby street). The tree is very old, remainders of an old iron grate have been overgrown by its trunk (so it is said in the city guide, it is not visible any more). Various exits lead out of this yard, even the one in the corner just left to the tree.
Don't forget to note down the last letter of the german name of this tree before you leave (index p).

NOTE: This place is described in various city guides, so its accessible for public. But when exploring please bear in mind that people actually live here.

Stage 12: The Game
N 48° 12.541', E 016° 22.576'
With a narrow lane in your back (where you have some nice examples of flying buttresses overhead) look at the opposite wall. Do you see what they are doing? I won't ask you the name of the game, that would be too easy. To the right of the amazing scene (somewhat below) there hangs a plate. Add the number of letters on this plate to the number of the house where the scene is painted. Convert the result to a letter (index a). If you are not sure about the number of the house don't hesitate to use the hint.

Stage 13: The Enemy
N 48° 12.560', E 016° 22.634'
Vienna is one of the few places (if not the only one) where this creature can be watched safely.
Below the writing on the wall there are two large words. Note down the counterpart of the first letter of the first as index n.
When has this house been renovated (ancient renovations don't count as their effects have been covered by others)? Add the first two digits of the year, convert the result to a letter and find the counterpart (e). The second digit alone will deliver index m.

A few steps to the right the last clue to the magic word is dangling above your head. Take the first letter of the english word of the object (index g), the second letter will be index j.

Stage 14: The Anagram

The magic password now has the format abcdefghijklmnopq. If that does not look like the location of a cache to you then play around with the letters and the place will soon be revealed. A good city-map will help you.
I admit this may be easier for the local people.

Stage 15: The Cache in Fang's Forest
The best way to get there is by public transport. The area is accesible at any time but there is more to see during daytime.
Find the eastern end of the waterfowl's playground. From there walk about 100 meters eastward and look out for a "junction". The cache is hidden under the northeast-corner. Kneel down and feel for it.

The area is slippery, be careful. Watch out for curious dogs and their suspicious owners (took me quite long to hide the cache). A stone is supposed to protect the box against mice..

Happy Hunting

The Cachebox is a little plastic container about 5 cm long, 5 cm wide and 3 cm high, there is no pen.

A short note on security:
The paved paths in the cache area may be slippery in winter. If it is icy or wet be careful (there are handrails).