**This article is about Tower Castles in general, and their use in adventures. It is not meant to be a technical discussion about such structures.**
"A tower castle is a small castle that mainly consists of a fortified tower, or a tower-like structure, that is built on natural ground. It is thus different from the motte-and-bailey castle, which it may resemble, but whose main defensive structure is built on a motte, or artificial hill. The tower castle is occasionally also described as a tower house castle or a tower house." ²
Technically there are several differences between a round tower and a square tower. A round tower offers more protection from siege engines, sappers, and projectiles. Square towers are easier to build, but their corners leave them vulnerable to mining. Unless your campaign uses advanced technical warfare rules, neither structure really matters for game purposes.
Many adventures seem to revolve around this type of structure in fantasy gaming. They are rather straightforward affairs, easily drawn up, and just as easily redressed to be used as a different location for later re-use. Often they are used in a gauntlet-style tier of encounters, with each successive level being more difficult than the last, until the Big Bad Guy (the BBG) is encountered at the top. This approach, however, has several drawbacks such as characters scaling the outside of the structure, or magic spells allowing characters to fly, thus skipping most of the encounters to confront the BBG. Often the whole party isn't present to help the characters that went on ahead.. leaving them without backup, so be prepared with a contingency plan for your adventure.
Tower castles are the most likely to have "dungeons" underneath them as they either sit on a natural rock foundation, or on solid ground. Motte-and-bailey castles have artificial mounds under them and underground construction would make them unstable, or at least limited in size. The legendary Tower of Zenopus, is an example of a tower castle.
Example Tower Castles:
|Image in the Public Domain - MacGibbon, D; Ross, T c.1887|
|Image in the Public Domain - An Inventory of the Historical Monuments in Essex, Volume 1, c.1916|
The Project Gutenberg EBook of British Castles, by Charles H. Ashdown, c.1911 - Has numerous drawings and sketches.
¹ - Source - Dictionnaire raisonné de l'architecture française du XIe au XIe siècle, c.1856
² - From Wikipedia - Tower Castle