NPCs - Troubles GONE

2. RESOURCES/TROOPS/DEFENSE
NPC city resources are replenish each 8 hrs, A barb city will take approx 1 hour to regenerate all to full (10% of the full amount per 6 minute tick). A barb city will also immediately regenerate approx 18% of all lost wall units each time some are destroyed. Example: if you destroyed 1000 towers and hit it again 1 second later 180 towers will be back.

3. ATTACKING
Attacking low lvl NPC cities are mainly for resource and gold, this is done with ballistas to achieve 0 loses. The maximum level without loses is level 5, level 6 or more have trebuchets so you will always suffer losses there.
But there are ARCHER TOWERS... so you need specific tech level of archery and Horseback Riding (HBR).

Archery 08 / HBR 03 (Massive losses due to being in tower range)
Archery 08 / HBR 04 (Smalls losses from units but @ tower safe range)
Archery 08 / HBR 05 (0 Losses )
Archery 08 / HBR 06 (0 Losses )
Archery 08 / HBR 07 ( Small losses )
Archery 08 / HBR 08 (Massive losses due to being in tower range)
Archery 08 / HBR 09 (Massive losses due to being in tower range.)

Archery 09 / HBR 01 (Small losses from units but @ tower safe range)
Archery 09 / HBR 02 (0 Losses when using over 575 ballistas )
Archery 09 / HBR 03 (0 Losses )
Archery 09 / HBR 05 (Small losses from units but @ tower safe range)
Archery 09 / HBR 06 (Small losses from units but @ tower safe range)
Archery 09 / HBR 07 (Small losses from units @ tower safe range)
Archery 09 / HBR 08 (0 Losses )
Archery 09 / HBR 09 (0 Losses )

Archery 10 / HBR 0 (0 Losses )
Archery 10 / HBR 05 (0 Losses )
Archery 10 / HBR 06 (0 Losses )
Archery 10 / HBR 07 (0 Losses )
Archery 10 / HBR 08 (0 Losses )
Archery 10 / HBR 09 (0 Losses )
Archery 10 / HBR 10 (0 Losses )

(copied from syoganai1111 thanks)

4. NUMBER OF BALISTAS TO ATTACK
This number depends on your hero ATTACK attribute, the safe numbers for heroes with no attack with 0 loses are:
(Note: You still need one of the above zero loss Archery/HBR combinations)

20 Ballistas vs Lvl1 NPC
50 Ballistas vs Lvl2 NPC
170 Ballistas vs Lvl3 NPC
250 Ballistas vs Lvl4 NPC
550 Ballistas vs Lvl5 NPC