NIB

Ninja castle attack. First get a castle next to your target castle. Then send 1-2 heroes with scout units while at the same time you attack the castle with a hero from your new castle. This way the enemy will only have a few seconds to react, not nearly enough to sell his resources, move his units, etc. Once you conquer target castle, sell resources for gold+buy scrolls+abandon castles+teleport home asap(recall back to abandoned castle).

This attack costs 40ap more but it gives 0 time to the enemy to react, allies cant team defend it and greatly reduces the chances of getting ambushed. Also you dont even need to use a lot of army, 500k-1.5mil should be enough to handle battlements(depending on the castellan and battlement level)

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Ninja reinforcement trick. Use the towers teleportation magic to transport units from secondary castles to your main in case your main or your portal is being ambushed.

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Instead of sending scouts from your main as stallers/nukers on a ninja castle attack, have some scouts on your ninja hero(the one who has the army and captures the npc castle) and once you conquer the npc castle, hire the 2 heroes from the tavern, give them scouts/scrolls and use them as stallers/nukers. Do your ninja castle attack, then remove scouts/scrolls from them and dismiss them. This way you draw less attention to you, it's easier to micromanagement(since you dont have to coordinate scouts that take 15mins to get to your target but only 20secs, so you dont forget them or mess up as easily), have less active heroes in the map for less time, etc.

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Send private message to your target to see if he is online. Note that he might be online but afk, many people leave the game logged in 24/7. Also note that this might tip him off about your upcoming attack.

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Use the cheapest 400dmg to all units scrolls to nuke enemy scouts/nukers. Always have 2-3 scout armies escorting your main force so you can delay enemies from escaping or prevent enemies from attacking your main army(attack delay them with scouts while your main army pushes forward or pulls out). And keep your scout armies between your army and the enemy main and/or direction of portal. This takes a little micromanagement since you have to keep stopping/starting your scout armies but it is worth since if you do it right, then there is little chance people can stop you.

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1. If an enemy is foolish enough to defend his secondary castle, it might be smart to first send a not so strong hero/army. That army should capable of killing at least 70% of enemy forces but the ideal is to kill a lot of enemy units but at the same time dont scare the enemy away. And once the enemy has lost a lot of units, send a second hero, as strong as possible with as strong army as possible, to beat the crap out of the enemy.

Now the enemy has 2 choices, either defend and lose all his army and the units in the infirmary, or retreat and just lose all the units in the infirmary :P. Now it might sound as stupid to sacrifice the army of the first hero but it actually isnt. Most of that army will go to the infirmary while 100% of the lost enemy army will be lost FOREVER. Note this tactic is pretty costly and risky for you as well, it's more of a theoretical kind of tactic.

2. The best way to defend a secondary, if you really want to, is to send an army from your main castle to the secondary(preferably with a scout command) and then stop that army outside your secondary castle and then manually attack the invaders. This way you can defend your secondary castle without worrying about losing units to infirmary bug since the lost units will go to the infirmary of your main castle. On the minus side, you need to keep watching that hero and actively attack enemy heroes.

3. When going or coming from somewhere, pay attention to the map. There might be ambushes, especially near the portals. Before starting a journey, check the travelling path for potential ambushes. Especially occupied mines or castles.