Green Dragons are special units in Stronghold Legends, and are only available to users who are currently playing as the Arthur faction. They are recruited from The Round Table, costing 1,000 gold and 250 Honour. You can find out ways to generate Honour in the game here.
You can purchase a Green Dragon at any point throughout the game, although you can only have one Dragon at any one time. Once it is killed, disbanded or lost, you can instantly recruit another one, assuming you have the required resources.
Unlike all other units in the game, Dragons take some time to recruit and you do not have access to them instantly. Once you have bought your Dragon, a green egg icon will appear on the right side of your screen, with a red line slowly moving clockwise around the outside of it.
This process is preparing a Dragon egg, and you will not be able to hatch your egg until the line is completely red. Once the egg is ready, you will be able to see this either by noticing that the red line is complete, or by checking your Round Table, as a large green egg will appear on the top of it when it is ready.
It is at this point that you may hatch the Dragon and use it in the game, at any time that is convenient to you. There are no time limits on when you can use your Dragon egg, and theoretically it could remain there all game until you need it. To hatch the egg, left click on the green egg icon, which should instantly turn into a Dragon icon. This will also instantly spawn a Green Dragon near the Round Table instead of the Dragon egg which was there previously.
Once activated, you have approximately 3 minutes to do as much damage as possible on the map using your Green Dragon before the countdown has finished and your Dragon is removed from the map. Since Dragons are so powerful in the game, they are time-limited in order to prevent them from easily overwhelming players.
Whether or not you actually do any damage is irrelevant, because the timer begins as soon as you hatch your Dragon, and you can see it counting down counter-clockwise on the Dragon icon on the right of the screen.
The Green Dragon can also be killed by enemy units or defences before the countdown is up. In this case, you will lose the remainder of your time and the Dragon will be removed from the map instantly. The Green Dragon can attack any military unit in the game, set fire to most buildings, and cause untold havoc on the map. Dragons are very durable and resistant to attack by almost any unit, except for attacks by Dragon Harpoons which can quickly deplete their health.
If you're attacking an enemy castle and it has stationed Dragon Harpoons on or around it, it's essential for your Dragon to take these out first in order to survive. Once the Harpoons have been destroyed, the Dragon can attack any enemy units as normal without the risk of quickly being killed. Dragon Harpoons can attack Dragons from a distance before they are in range to attack the Harpoon itself or anything else nearby, so it's important you minimise this damage.
Green Dragons can also attack other Dragons. If the enemy spawns a Dragon of their own your Green Dragon can easily attack it while it is on the move. Unlike ground-based units, because the Green Dragon is an aerial unit, it can fly through the air and therefore travel over water or other impassable parts of the map by land. You can use this to your advantage to significantly reduce the movement time of the Dragon, and to gain access to other areas which are difficult by land to take out crucial defence systems.
In straight out combat between a Green Dragon and a Black Dragon, the Green Dragon will kill the Black Dragon with a little over a third of its health remaining. It is also totally immune from freezing so the Ice Dragon will have no effect on it whatsoever. The Green Dragon can however still attack the Ice Dragon, so the Ice Dragon will be killed with the Green Dragon keeping all of its health. This makes the Green Dragon the strongest and most effective Dragon in the entire game.
Please note that Green Dragons, like all aerial units, are still susceptible to damage by fire on the ground, even if it is their own fire. It's therefore best to keep your Dragon moving around and keep them out of fire areas to protect its health. If you're feeling lazy, you may even wish to attack an enemy Lord using a Green Dragon.
A Green Dragon can easily kill an enemy Lord within its time limit, and if you're struggling with your troops or defences then this is a potential option, although it is a relatively easy way to win a game and is not really recommended in practice.
Tip: Dragons are one of the supreme units in Stronghold Legends, and are virtually untouchable by all other units other than Dragon Harpoons. For this reason, you can cause immense damage to the enemy, particularly if you attack melee units on the ground with no nearby ranged units to attack you back.
For best effect, use your Dragon to quickly cause damage to an area and then move on to a different area before the enemy can mobilise their troops or any defence. You can quickly destroy an enemy economy by doing this.