Akasaka is business and commercial district located between Aoyama and Roppongi and Edo Castle. Due to its elevation and proximity to the castle (ie; right next to it), it was the site of many residences of those daimyō who were the closest to the Tokugawa shōgun, including other branches of the Tokugawa clan. Today it is just one train station away from the 国会議事堂 Kokkai-gijidō National Diet Building. The area has long been a meeting ground of the rich and powerful, and even today it is one of the few areas where you can still find geisha in Tōkyō. The crown prince’s residence is in Akasaka, as is one of the former imperial villas (now a guesthouse for visiting heads of state).
The word is made of 2 kanji:
赤 aka red
坂 saka hill, slope
There are two explanations. A third explanation is just a combination of these two.
The first explanation is that the slope had ruddy clay that gave it a distinctive look. The second explanation is that the hill was formerly referred to as Akanesaka or Akaneyama (written赤根山 Akaneyama or 赤根坂 Akanesaka or 茜坂 Akanesaka). 茜 akane Japanese madder or 赤根 akane Japanese madder (literally “red root”), is a plant used to make a brilliant red dye (from its root). Supposedly the area was famous for this plant and it was easily collected there before the arrival of the Tokugawa. Over time, the pronunciation became slurred and Akanesaka turned to Akasaka. The 赤根坂 Akanesaka variant could easily be reduced to 赤坂 Akasaka. The third theory is a mixture of those. It points out that the roots of the Japanese madder, being used for making red dye, naturally turned the dirt and clay of the slope red. Therefor the hill was known for a plant already associated with red and a hill that had reddish dirt.
To be honest, I’m not sure if I believe any of these theories, but the third theory does a good job of tying up the first two. Since the area doesn’t have any Japanese madder growing anymore – and I haven’t seen any red clay there – let’s just say the jury is out on this one until we get more evidence.