It would be amazing if you added an Ice Trap. When the enemy gets on it it will slow them down. You can upgrade it to get to a max of 50% starting from like 5% or something like that.
I was surprised the Spikes includes a stun function. I sort of expected that to be a different building. So Spikes only does damage, and Leslies' Ice Trap does the stun part. I was going to suggest a Tar Pit for that purpose.
So my idea is: split the Spikes into two buildings: Spikes and a stun / slowing down building. So in any given square one can only build one or the other.