The ordinary meaning of nap is to doze off during the day. The secondary meaning is to be offguard. Hence the phrase Catch them napping . Flying by the nap of the earth refers to the practice of flying low to the earth (undetected by radar) to catch the enemy by surprise (or napping).
the tribunes The Tribunes were elected representatives that protected the plebeians from mistreatment. The Tribunes attended meetings of the consuls or Senate in an attempt to bring justice to the plebeians.
The Colosseum was covered with an enormous awning known as the velarium. This protected the spectators from the sun. It was attached to large poles on top of the Colosseum and anchored to the ground by large ropes. A team of about one thousand men was used to install the awning.