All Havdala Bsamamim boxes I have ever seen are in the shape of a tower.
Why (and when) did it take on that classical shape (rather than just a silver box)?
This article, by Rabbi Shubert Spero, says that the fashion started around the sixteenth century.
He suggests that a tower symbolizes strength and security, and so it fits well with Havdalah, in which we speak of Hashem as our source of security ("כי עזי וזמרת י-ה"), and also that it is associated with the rebuilding of the Beis Hamikdash and the coming of Eliyahu and Moshiach, a major theme of Motzaei Shabbos.
Spices tended to be expensive. Expensive things were kept in a vault, often located in a high tower (if one owned a castle...). The day-to-day experience of Medieval Jews made those towers a natural model for b'samim.