Added keyboard shortcuts for buying towers.
Explored the source destruction problem though restructuring. Fixed the lone-source destruction problem, but am (almost) back to the old dropped-source destruction problem. Almost, because I can now discern a reason for the problem. However, this required me to destroy the source in the update, which was something I was considering previously, anyway.
In the restructuring, I can, however, see the problem: somewhere between when the source is inserted in the destruction queue and the destruction of the source, the site is inserted placed after the source in the queue (removing the ground tile in the process, I suspect). Possibly, before I reordered the code, it was putting the source after itself in the queue.
If I can make sense of that, then I'll have a game.