Dragon Age: Origins
77 Achievements
80-100h
Defender
Preserved the lives of half the troops at Denerim's Gates in "The Final Battle" Details for this achievement will be revealed once unlocked
28.0%
How to unlock the Defender achievement in Dragon Age: Origins - Definitive Guide
During these battles you have to ability to summon armies within the 'Advanced' section of the Radial Menu. To unlock this achievement, you must keep half of these armies alive and it unlocks upon arriving at Fort Drakon if I remember correctly. During the Archdemon fight you can use the armies as much as you want.
The most certain way to get this is to just not call out any of the armies. It's not even necessary on the easier difficulties anyway.
*Side note* I got this by using the golems at the market, elves at the alienage, and the arl's soldiers at the palace on normal difficulty
I made it through the alienage and market only losing 5 total troops, and the cheevo dinged after Fort Drakon loaded up.
During "The Final Battle", you'll gain the option to call in support troops (, then select Advanced, the choices are listed on the bottom of the radial menu) from the human, dwarven, or elven armies, or a group of mages. The total will vary depending on who you side with. As long as more than half survive by the time you reach Fort Drakon, this will unlock.
Strategy: The Market District is easy to clear without any armies. For the Alienage, a single set of 50 should work out for you. This should be enough to keep the heavy wave occupied while you slaughter them. Be sure to take out the enemy mages first, then the general. For the Royal Palace all you'll need is your 6 golems. You should easily be able to make it with more than half your warriors intact.
Though if you want to use your troops, there is a way to keep them alive. You just need to level them up. The emissaries at your camp /do/ serve a purpose, and when give the correct items, will level your troops up.
* Golems - are at max level and don't require any items (4 troops, lvl 15)
* Werewolves - are at max level and don't require any items (16 toops, lvl 12)
Dalish - Elfroots, deep mushrooms, metal shards, deathroots - Max troops 50 + Max lvl 9 - 10 xp per item
Redcliffe - Money - Max troops 50 + Max lvl 7 - 10 xp for 50 silver; 42 xp for 2 sovereigns; 225 xp for 10 sovereigns; 750 xp for 30 sovereigns
Dwarves - Sapphires, Amethysts, Malachites, Topazes - Max troops 50 + Max lvl 9 - 10 xp per gem
Mages - Runes (no grandmasters) - Max troops 12 + Max lvl 10 - 10 xp per rune
Templars - Runes (no grandmasters) - Max troops 16 + Max lvl 12 - 10 xp per rune
There is no good way to tell if your troops have been leveled up. Some people like to go by what the emissaries say after you've deposited items, but it's not conclusive.
(some info taken from the da wiki)
During "The Final Battle", you'll gain the option to call in support troops (, then select Advanced, the choices are listed on the bottom of the radial menu) from the human, dwarven, or elven armies, or a group of mages. There are 162 total. As long as more than 81 survive by the time you reach Fort Drakon, this will unlock.