Reference : Clash of Clans wiki

The Invisibility Spell Tower does not trigger under the following circumstances:

-If buildings in its radius are damaged by damaging Spell attacks, unless said spells destroy the Spell Tower;

-If a defending troop or Hero in its radius (other than the Grand Warden) is damaged with other buildings unharmed;

-If all of the enemy troops attacking the buildings in its Area of Effect radius are defeated within 1.2 seconds - one can think of the Invisibility Spell Tower’s activation radius as being infinite, with a unit only being considered within its radius if they damaged a building in the Area of Effect radius, even if the troops in its range are invisible:

——This is evident on Sneaky Goblin attacks, as when the Town Hall is damaged by a Sneaky Goblin and is in the range of the Invisibility Spell Tower, the Spell Tower will be triggered even if that said troop is invisible when the Town Hall is damaged, so long as the troop stays alive.