To make this a bit more programmable, why not simply place them automatically on the first open creature slot on the opponent's field? As far as the game stands currently, this has the same effect anyway, plus no danger of misclicking / choosing the wrong spot.
Also kind of just a reference, but real-life weaponized spikes (at least the only I know of) are called Caltrops (
https://en.wikipedia.org/wiki/Caltrop).
Thematically I think a minefield / mine is more appropriate, but I'm just picking hairs now.
This is a great idea. It could force your opponent to hold onto their creatures until they draw one strong enough to take the hit, or can play two at once. Oty might make a good pairing to cover the creatures with enough HP to survive Tacks|Spikes.