Pay careful attention to the location of Gold Mines when planning your attack, as Gold Mines are usually only marginally protected and can be quite lucrative, especially at high levels. Many times a single Archer can be placed in such a way that it can destroy a Gold Mine while standing clear of any defenses.
If you see a round number of gold in a raid, it is most likely going to be in the Gold Mines rather than in the Gold Storages (example 90,000). Although round numbers can occur in Gold Storages, they happen much more often when Gold Storages have been emptied by a previous raid and the resultant shield has allowed the Gold Mines to completely fill up.
Make sure you target carefully, as gold mines tend to lure your troops away from what you really want to attack, ex. a Cannon or the Dark Elixir Storage.
Gold Mines undergo significant visual changes at levels 5, 8, 10 and 12.
When initially constructed, the Gold Mine consists of a somewhat dilapidated wooden structure partially covering a set of narrow rails diving deep within the earth. Periodically a rail cart full of gold arrives and dumps its contents into a small repository to the side of the mine, and then re-enters the mine.
At level 2, the Gold Mine appears to be fully repaired, and two wooden reinforcing bars appear on the outside of the wooden structure. There is also a very small stone rubble pile on the edge of the structure.
At level 3, a third wooden reinforcing bar is added, and the rubble pile outside the structure grows to a noticeable size.
At level 4, a fourth wooden reinforcing bar is added, and the rubble pile outside the structure grows larger as well.
At level 5, the first major graphical change takes place. The wooden reinforcing bars are replaced with a blue-gray steel bar, and the rubble pile outside the mine is replaced by a stone support.
At level 6, there is a very slight and rather unnoticeable change. There is a small wooden reinforcing bar added to the back of the Gold Mine.
At level 7, the steel reinforcing bar appears to be larger in width. Another wooden bar is also added to the back.
At level 8, the Gold Mine now has two wide steel reinforcing bars bolted down, and a small pulley system, complete with wooden track for the cable, appears on the top surface of the mine.
At level 9, additional pulley hardware is added to the top of the structure, and the pulley wheel itself grows larger.
At level 10, this is another large graphical change as the Gold Mine now has a large cover on the top made of steel that covers the wood below, and the sides are more heavily protected by spiked metal. A large gear appears beside the pulley wheel as well.
At level 11, the pulley system on top of the mine grows larger still with a second large gear and pulley wheel appearing, and additional spiked metal armor appears around the sides.
At level 12, the pulley system on top of the mine becomes gold, and the corners and top of the gold mine turn black. Also, the bolts appear to move higher above the spiked metal armor.
It takes 15 days, 21 hours, 20 minutes and 10 seconds to fully upgrade the Gold Mine.
A downside with Gold Mines is that even though the cost to upgrade them is low, the time it takes to complete them is relatively long.
* The Catch-Up Point is the time at which the newly upgraded Gold Mine has equaled the total production of the mine had it not been upgraded - this does not consider recovering the cost of the Elixir required to perform the upgrade.
Caveat: The Catch-Up Point calculation assumes that the player collects all resources from the Gold Mine without allowing it to reach its storage capacity, as resource production ceases at that point. This is more of a concern at lower levels where the capacity is limited and Time to Fill is relatively short. If the player is unable to regularly collect and achieve the full potential of the collector, the Catch-Up Point is actually shorter and performing the upgrade becomes more advantageous.