The Creep Tower's healing rate is proportional to the current difficulty level. It heals for 200HP/s on Normal, 250HP/s on Hard, and 300HP/s on Insane and Mental. It is highly recommended that the Creep Towers near a Hive be killed before the Hive itself. Tac Nukes are generally an effective way of doing this.
Creep Towers have an increased AI priority, making CP forces, your minions and your hero (if you attack-move) to prioritize attacking it.