Do structures count as terrain?

Question

Vayne's Condemn says:

If they collide with terrain after being knocked back, they are dealt the same amount of physical damage again and are stunned for 1.5 seconds.

Similarly, Poppy's Heroic Charge says:

... if they collide with terrain, her target will take a high amount of magic damage and be stunned for 1.5 seconds.

So if the enemy hits a turret, inhibitor, or nexus, does it count as terrain and cause these abilities to stun the target?

Answer 1

Yes. Turrets count as terrain for the purpose of champion abilities.

The wikia confirms this:

Towers count as terrain. Any abilities that interacts with terrain will trigger the effect, namely Poppy's Heroic Charge's and Vayne's Condemn's stuns.

Answer 2

Yes, they count as terrain. They even count when destroyed. So do things like trundle's pillar. It can be very powerful for trundle to cause a pillar to appear behind the target, then for vayne to shoot him.