Welcome back! If you are reading this, you are either here intentionally or the Google machine sent you here by mistake. Either way be sure to sit down and strap in because this is going to be an exhilarating experience!
So my fancy book readings told me that due to the magical properties of concrete, you should build a box out of rebar and pour the concrete around it. Concrete has great strength in compression, but doesn’t have nearly as much strength when tension is applied. To remedy this, we add the magical properties of steel. Steel is STRONG in tension, so by building a box out of the stuff and pouring concrete around it, the steel takes the tension and “the crete” takes the compression. By putting some blocks/bricks/books/bills under the rebar cube, it will ensure that the box sits off the gravel base and no rebar ends extend out past the edges of your concrete cube. This is important so that moisture doesn’t rot away the rebar over the years, thus compromising your tower cube and possibly the tower’s structural integrity itself.