I think the problem with the Tshike is that it comes too late. I can obsolete it within a few turns of getting them. I'd like it if they replaced ancient walls and automatically upgraded to the defense of the highest level available. So it you researched castles, they'd increase in defense. Also make it available at mining instead of masonry. And before anyone cries overpowered, this is the same game with Nubia and Sumeria in it.