Capturing a city can take a little bit of getting used to … the key is to not panic. You may be tempted to throw everything you have at the capture but unfortunately, all you will do is suffer significant losses and slow down the rest of the team.
I will try to explain this the best I can but remember if you don’t know, it pays to ask, there are always people in an Alliance that want to help you grow.
There are 2 parts to taking a city that need to be done within 30 mins or the protectors will come back and the whole process starts again. –
The Garrison
These are the protectors their job is to stop you from getting to the wall to break it down. Their strength differs depending on the level of the city you are attacking. You will need to ensure that your team can beat these guys as quickly as possible. There is no use sending a team that cannot achieve this all you are doing is eating into the time available for the bigger guys in your alliance to kill them off.

You will see this city is a level 4 –
The protectors ( 1 ) are level 75 they are backed by level 7 troops (2) you must be able to beat this team quickly to be of assistance to your Alliance.
You should always rally when attacking a city as it helps to boost the overall damage done. Again time is of the essence here and you really are better to sit back and watch than fill up space on rallies that should be taken by those that can beat the garrison quickly.
Your alliance will need all the time they can to take down the wall especially since as the level increases so does wall size but the time allocation does not.
You will need special fuel to attack the city, you are provided with 15 of these to use that refill over time at a rate of 1 every 2hrs, so they are quite precious. You can also purchase extra from the merit shop, I buy these every week to keep stock in my inventory for those extra tough city attacks. You can buy these with real money but you do not have to … Please always play within your means it is after all just a game.

The Wall
The wall (3) on this level 4 city has 40 000 hit points, this will change according to the level of the city, you will need a destruction team to break this efficiently. The 4 heroes below are the only ones that deal mass destruction to the wall (at the time of writing this)

You will need to make sure the 3rd skill is available on your heroes to ensure that the bonus destruction damage is available

Build one truck with as many of the destruction heroes you have available in it, then fill the remaining spaces with your biggest damage dealers.
During the assault only send this truck, slow and steady wins the race. Do Not send every truck you have as they won’t deal anywhere near the damage needed and all they do is increase the queue length for the other alliance members.
The longer this takes the less time your team has to take down the wall. Please note: the only time it is ok to send other trucks is if the destruction on those trucks is above 200.
Destructive might is the single most important number when it comes to the wall.

It pays to stock up on the destruction bonus item in the merit shop.. This will be available after your Alliance captures a level 5 city at a cost of 100 merits. I always make sure to buy them because when the season ends the merit shop resets, these destructive bonuses are a big help for early in the new season.

Immunity
Immunity (4) this is needed to attack a city, without it you will suffer massive losses as a percentage of your troops will be killed instantly. Your immunity is built up over time by capturing mines and using the materials to upgrade clinics, these clinics in turn provide you with immunity. There is more information on this process on the blocks page.