It wasn't JUST a ninjitsu school. In fact, it wasn't a school at all! There were under ten pupils (my uncle being one of them) being trained and taught by a ninjitsu master. As in he came from a line of ninjas. He might have been a descendant to ninjas, or might have been taught by them. Either or, he taught the history, techniques, and thinking of a ninja. Therefore, my uncle is basically an expert on the subject.
So did his heritage mean that he qualified as a ninjitsu master, or was he himself trained? And if so, where did they get their information from? And for him to be able to trace his line from ninjas, there must be written evidence of a ninja's existence (or more than one oral story about it). If there is no written evidence or a single oral story, then how can you be sure that he could trace his line from ninjas?
Yeah, I'd say that it is possible. Especially swinging from building to building in a city. The castle would be a little tricky. But either take out the guards or climb up the wall when they aren't looking. Not impossible, but very tricky.
Climbing a wall quietly is difficult when you have a lot of equipment (none of which is that practical for climbing walls), and when there are likely to be braziers at the top. If you threw anything at the wall of a traditional Japanese house, there are several things that could happen: the noise could be heard as the walls were thin, the object would simply bounce back at whoever threw it because of the circular construction of the wood, or the wall would break.
Ahem. I said shorter bows. As ninjas used them every once in a while. Not the yumi bow, but a collapsable, or short bow.
There is no record of such weapons in Japanese history. Most of the eastern empires had compound bows, which were definitely not collapsible. Those that did not had longbows, which we've established could not have been the weapons used by ninjas.
Um, I'm thinking of bushes, boulders, trees, cielings, and with the help of the shadow of night.
Japanese gardens were lit with torches and braziers, and if they weren't the water features would have been invisible and would have betrayed a ninja's presence by the noise of their feet. Even if they hid behind something, they would have a shadow.
The Empire is a confidence trick. Do you, the famed warrior races of Tamriel, having won international renown for your fighting prowess, consider yourselves and your abilities inferior to those of money-grabbing merchants and diplomats in armour?