League-of-legends – Can Fizz jump over Veigar’s Event Horizon without being stunned

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.

Untargetable refers to champions in a state where targeted spells cannot be cast on the untargetable champion, and nontargeted spells such as Karthus's Requiem will have no effect. Additionally, they cannot be targeted by auto attacks.

A more comprehensive list of Invulnerability:

Complete Invulnerability

These break all forms of leashes and makes the champion invulnerable to all damage.

• Shaco's Hallucinate makes him untargetable for a short time before the clone spawns.
• Zhonya's Hourglass's active ability makes your champion untargetable for two seconds.

Partial Invulnerability

Projectiles fired before the untargetability starts will have no effect. They will still take damage from DoTs such as Ignite and Malzahar's Malefic Visions and will not break leashes such as Fiddlesticks's Drain and Morgana's Soul Shackles. They will still be damaged by Zilean's Time Bomb, but not Fizz's Chum the Waters.

• Fizz's Playful makes him untargetable for 0.75 seconds or until he uses Trickster.
• Vladimir's Sanguine Pool makes him untargetable for 2 seconds.

Dashes

Projectiles (autoattacks or abilities) that are mid-flight when the character starts dashing will hit when the dash completes. They still take damage from DoTs while untargetable.

• Maokai's Twisted Advance makes him untargetable while moving to the target.
• Master Yi's Alpha Strike makes him untargetable for a short time while in flight.
• Fiora's Blade Waltz makes her untargetable for a short time while in flight.

source: http://leagueoflegends.wikia.com/wiki/Untargetable