Falling Spikes are typically hidden in a row of other upside-down spikes lining a castle ceiling. When Mario approaches, it begins to shake, then falls straight down, damaging Mario if it lands on him. Falling Spikes can be tricked into falling early by scrolling the screen to the right. They are slightly darker then normal spikes.
Falling Spikes are sprite B2. They use Mario's palette 8. Their graphics will only show up correctly if the castle sprite set is used. (SP4=03)
Falling Spikes cannot pass through a slope, so plan accordingly when designing levels that utilize them.
Related ROM Addresses
0x1941C [E0] Falling Spike tilemap