There are really only 3 types of heroes too look for in the game that are essential to a long term active player:
1st) Governors - These are generals with high politics value nad preferably farm bonuses attached to there automatic skills slot. Thankfully we all get one at the very start of the game, LiuBei/SunQuan/CaoCao examples. - BTW Cao Cao has the highest politics value of any general in TKO. Keep that in mind when upgrading him and assigning him to a station. Xun Yu/ Sima Yi/ Pang Tong, these are also good examples of governors.
2nd) Attackers - Attack generals come in many shapes and sizes, Infantry/Cavalry/Seige - these are all types of generals that we would like to have 1 of each in every attack city we have. However you will find that searching for epics can get quite costly so I try to stick to looking for one that can win duels. Example: Pang De/ Xu Chu - these are the best 2 dueling generals in the game. They are the only two generals in TKO that have 799 Hit Points without any bonuses added to them. If your attacking general can defeat the defending general then your army morale is boosted during each assault. That boost might be enough to give you the edge.
3rd) Deffender - This is a hero with high leadership value and or Vacancy ability. Zhou Yu/Zhuge Liang - These two combined would be ideal however very unlikely unless you are going to use ALOT of gold. Zhou Yu is the only general in TKO to have a leadership value of 100 without any upgrades. Zhuge Liang is the only general in TKO to provide you with a 50% Vacancy ability. This means 1/2 of your attackers will be turned away without any type of conflict.
Hope this helps some
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