On page 24 of the playbook, describing Belgic non player free Rally, it states how the first thing you look to place is a Citadel, which according to the playbook, is not possible.To place a citadel you need an ally in a city spot, those are the allies with a square outline around them, e.g. Cenabum in the Carnutes region or Alesia in the Mandubii region. See rule book page 3 section 1.3.3 for a picture of one.
Why is it not possible?
My understanding for Rally is that to place a Citadel, all you need is an Ally already on the board, which the Belgic have several. So why do I not replace an Ally with the Citadel for the free Rally?
The Belgae do not have an ally in such a spot in the example.