If the construction started in 1940, the situation in Germany was completely different from what happened there 3-4 years later! I bet no 1940 type bombs would do much harm to these towers.If we add all this info together as an Ally bomber that wants to demoralize the population with terror and death I would simply just pound as much as I can the towers
Flak was often placed on house roofs, or special made towers if there were no suiteable hills nearby, so a Flaktürm would solve 2 problems with one construction...Both shelter and platform!
EE