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 00 (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 )
NUMBER OF BALISTAE 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 Ballistae vs Lvl2 NPC
170 Ballistae vs Lvl3 NPC
250 Ballistae vs Lvl4 NPC
550 Ballistae vs Lvl5 NPC