Turtling is a common nickname given to a defensive strategy which involves avoiding the opponent, using ranged attacks, and punishing bad approaches. It is normally used when one is holding a small lead in a timed match, where it can reduce the timer significantly while not having much effect on the match, or even be used until the timer ends. Flat stages without platforms, especially Final Destination, are preferred for maximum usage of this technique, as it makes it harder for the opponent to approach, particularly if they lack a projectile, have poor mobility and/or range, or have large hurtboxes.
A common characteristic of characters that are especially good at turtling is that they often tend to have projectiles that leave them briefly open just in time to retaliate any attack that comes after them. They also tend to have attacks with good reach and short startup or ending lag, as well as rather high air speeds that allow them to move around quickly when jumping. While it can be done with many more characters, examples of characters that can take good advantage of this strategy in Ultimate include R.O.B., Samus, Robin, Pac-Man, Min Min and Villager.
Air dodges, quick air speed, quick ground mobility, and attacks that easily allow a cross-up (such as many dash attacks, some down tilts, and several special moves) can all help break turtling by forcing a close-range encounter.