Five Norman knights had already been chosen by the Prince (пять рыцарей-норманнов уже /заранее/ были выбраны принцем) to be the 'challengers' for the day (быть "зачинщиками" турнира /в тот день/;
signal ['sɪɡn(ə)l], herald ['herǝɪd], rule [ru:l], challenger ['ʧælɪndʒǝ], shield [ʃi:ld]
Prince John gave the signal for the tournament to begin, and a herald came forward to read out the tournament rules. Five Norman knights had already been chosen by the Prince to be the 'challengers' for the day. These five knights stood next to their tents at the end of the lists and held up their shields for everyone to see.
According to the rules (согласно правилам), these five challengers would have to fight (эти пятеро зачинщиков должны были сражаться) against any other knight (против любого другого рыцаря) who chose to fight against them (который захотел бы сражаться против них;
If he touched the shield with the back end of his lance (если он касался щита тупым концом своего копья;
according [ǝ'kɔ:dɪŋ], touching ['tʌʧɪŋ], covered ['kʌvǝd], rider ['raɪdǝ]
According to the rules, these five challengers would have to fight against any other knight who chose to fight against them. A knight could choose to fight against one of the challengers by touching that challenger's shield with his lance.
If he touched the shield with the back end of his lance, the two knights would fight with their lances covered, so that there was no danger to the horses or riders.