This achievement can be only unlocked every three weeks during an INVASION event. (Every week you either get Guild Wars, Raid Boss or Invasion)
You need to be in a guild to enter the event!
Check the Guild screen to see when the next Invasion event starts.
To win the achievement, you need to destroy 50 towers in one week. (Towers are special troops appearing in the Invasion event only. )
To fight in Invasion, you create a team comprising of a particular troop type. In the first invasion (starting March 5), for example, you can only use beasts. You can also use your hero (no matter what class you are).
You fight battles in 8 stages, each stage having 5 battles:
Stage 1: Village – no towers
Stage 2: Outer Keep – 1 tower in each battle
Stage 3: Inner Keep – 1 tower in each battle
Stage 4: Outer Castle – 2 towers in each battle
Stage 5: Inner Castle – 2 towers in each battle
Stage 6: Outer Fortress – 3 towers in each battle
Stage 7: Inner Fortress – 3 towers in each battle
Stage 8: Citadel – 4 towers in each battle (not 5 battles but 65 battles + increasing difficulty!)
(you'll need to play over 31 battles)
To fight these battles, you need sigils, one sigil per battle. How to get sigils:
- you receive 4 sigils for free each day (you can wait for the last day of the Invasion and use them all or play a few battles every day)
- you can buy them in the Invasion shop for gems
- you get them from Valravens (troops appearing randomly in these battles), 2 sigils per Valraven. Valravens are fairly common, appear in like every other battle. Just kill them before they fly away. When you get to Stage 8 (whih you don't need to, for this achievement) there are no more Valravens. Instead, you get 2 sigils every now and then for winning a battle.
Unused sigils from Raid Boss do not carry over to Invasion (or the other way round). Sigils expire at the end of the event!
Like in the Raid Boss event, you can buy a special troop in the event shop, one that is extra strong against towers. This week it's Stone-Shaker. Using this troop is vitally important to win higher stage battles.
Before Invasion, I suggest saving up at least 250 gems for the first three tiers of shop rewards. You will get the special siegebreaker unit and a special hero weapon that works well with the given troop type. Also you will get extra sigils, traitstones etc.
The achievement unlocks upon completing the battle in which you destroyed your 50th tower. (You can lose battles, destroyed towers still count.)
You can see how many towers you have destroed in the Overview tab of the event.