After extensive testing, I have determined all the sources that can increase the HP or damage of Rooks towers. They are listed below:
That's it.
They do not receive bonuses from citadel upgrades.
They do not receive bonuses from +minion effects or any other item properties.
They do not receive bonuses from the tower strength setting during level creation.
At level 10, with four points in the skill, they're as good as they're ever going to get.