Slows down dragons that are on top of it.
Bronze has slowdown of 63%
Silver has slowdown of 76%
Gold has slowdown of 89%
It's best to not put all your walls in the same row. This way dragons will reach different rows at different times. This is so that your cannons can start shooting some of the dragons soon, then more will come that were slowed down.
Place your best wall in the same row as your best cannon to get the most usage from your cannon. Also put your best arrow tower next to it.
Dragons like warmer temperatures (they breathe fire). Ice gives off cold air and makes things move slower. The dragons in Tower Swap cannot fly very high.