A good Clash Royale deck is defensively strong if it can defend against various attacks and has good synergy between the cards23. Here are some elements that contribute to a deck's defensive capabilities:
A deck's defensive potential is calculated by looking at the defensive cards and their synergy2.
A good deck should be able to defend against swarms, tanks, and specific units2.
Having more than one reliable anti-air unit is safer because one can be removed by a spell. Insufficient anti-air units can cause problems when dealing with certain cards2.
Buildings are commonly used for defense against units5.
Cards that work well together on defense make a deck more effective2.
A good deck needs enough flexibility to defend from the opponent3.
Cards that remain effective even at lower levels are beneficial for a deck2.