Advanced format ruling
Kaiser Colosseum
Card effect (current official text)If there is 1 or more monster(s) on the field of the controller of this card, his/her opponent cannot place a monster on the field if his/her number of monsters would exceed the number of monsters that are on the field of this card's controller. The cards that are already on the field before this card's activation are unaffected by this effect.
Current rulings for this card are maintained by Konami.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Advanced forum — sourced change requests get reviewed.