If I'd made it a prototype I think I'd have saved myself some effort and carried on building on top of it.
It's very flat on top - was Silbury always so flat on top? (impossible to say I suppose. But I thought it had been flattened somewhat by later people who used it as a lookout or whatever. It would be useful to have flat-topped if you were going to sit on it to look at something from. But if it's just to look at and not meant to be scaled (no reason why this should be so?) then you'd prefer a nice rounded top would you?)
