Constructing a Tower is a Legends of Idleon quest: 2 rewards. Build the Red and Orange Signposts, and also make some Cogs by going to the 'Cog' Tab and opening the Cog Shelf and dragging in some players.

Constructing a Tower | Idleon Toolbox

Build the Red and Orange Signposts, and also make some Cogs by going to the 'Cog' Tab and opening the Cog Shelf and dragging in some players.

Constructing_a_Tower

Hold down to add these gems to your account. You can then spend them in the Gem Shop!