Instead of going in and out every few seconds, spikes should go "on/off" without actually showing their pointy ends.
They'll still show an animation as a visual cue, you'll only need to pay more attention.
If a knight steps on them when they are "on", they get impaled.
Dashing could prevent them from poping.
What if I want to have a tea party on the floor whilst getting a spike up the backside every few seconds? Does the floor only spike me once then retract? Or will it just keep its spike up for as long as I'm on it?