I started a Bushcraft and Nature social group on the Scouting forum called Escouts some years ago and despite a healthy three figure membership it was by and large just me posting...almost my own blog, hence the reason I am now putting my efforts into this one instead!
Early on I tried to get some content on there double quick, and one of the earliest things that I put on there was a tower fire. I seem to recall Bear Grylls making one on an episode of Born Survivor.
You'll need to start by selecting your green sticks about a foot or so long, this is ash (from this blog page...The gift that keeps on giving) and about the same diameter as a ten pence coin. I've propped up the Bahco folding saw for scale. The ground was soft and it was fairly easy to baton the four sticks in (about one and a half to two inches apart) as shown. If the ground is harder it may be worth shaving the edge off the circumference of one end of each column to prevent splitting.
Before you do anything else, check that the vessel you wish to boil water in fits safely and that the sticks are level. Next the spaces between the columns need filling with small stick fuel on top of some birch bark and dry bracken or similar. As the ground was wet I placed a few very small sticks flat at the bottom to lift the fuel away from the moisture and mainly placed the fuel at ninety degrees with the odd diagonal layer.
Once the space is all loaded to the top, place your vessel on the top and light the tinder, preferably on more than one side. The flames soon spread and the only time they are a little less effective is if the wind is strong because the flames blow sideways, as opposed to up the column.
As a rough timing the steaming picture (above left) was achieved after a little over five minutes but of course the weather, fuel, liquid volume, height etc will effect the timings. Tea was achieved a short time after that with a rolling boil and yes, I've transferred it to the plastic equivalent of the metal Crusader cup so that I retain some lips after drinking the tea!
I had planned to pull the columns out straight after a brew was achieved to see how much integrity they had left in them, I decided instead to see how long the structure stayed viable and I'd estimate around twenty minutes tops with the wood used before the final column collapsed which is why it's only really suitable for a brew and that you need to keep eyes on when using one.