This is an example of a basic battle tower for reference as part of an article I've published, The Art of Vanilla PVP.

The design is strictly for functionality and resource efficiency. It requires 76 cobblestone and 8 obsidian.

If anyone would like to make recommendations or demonstrate other effective combat building designs please feel free to post the links to your schematic here in my comments or on my article. As always I love a good discussion on combat tactics in the context of building design! :)

I should note, although cobblestone is hideous looking in my opinion, it has a slightly higher hardness of 2 as compared to the more aesthetically pleasing smooth stone or stone brick with a hardness of 1.5.