Fully upgrade your gem runner first. Send him for diamonds to sell. It's the best way for cash. You just need to pop in and out for them to return. No time limit is required.

The tower upgrade is based on the quests of the NPC's.

UNKNOWN: Friends help you move...True friends, help you move bodies...
E.A.P.: Blood was it's Avatar and it's seal.
E.A.P.: Stupidity is a talent for misconceptions.