Before we talk about base building, you need to unlearn every habit from standard Ranger play. Calliope is a with the following base stats (un-upgraded):

This is her "DPS" floor. Calliope relies on volume. Because her individual shots deal less damage than Caelus, she needs to fire fast enough to prevent "runners" from reaching her. Movement Speed:

Calliope’s stun + Stone Tower’s slow = infected moving at a crawl. You can reduce a harpy’s charge to a walk.

A "better" Calliope build has 320 HP, 120% attack speed, and 50 base damage . This build clears infested villages faster than a squad of 10 Rangers.