3. "expression of power" means they could build a wall so they did, to show the amount of power they have. The "expression" is showing that they have the power to do whatever they want.
5. What suggests that the Romans were not the only people who avoided admitting 'out Loud' that they wanted to collect taxes?
The statement "how little things have changed" shows that many countries, from Ancient Rome to present do not want to admit that they are mostly motivated by money and use taxes to get more.