You need to stack thirty castle (stone wall) blocks, reaching 31 total health in the stack. As walls can only be stacked three blocks higher than adjacent ones, this requires at least a 2x1 pattern. It is possible to pause the game, create two stacks of 27 and 30 height adjacent, select the 27 height stack and cancel it (delete key) before unpausing. This way you can place a single 1x1 tower with no extra cost. As the achievement does not require building an archer tower on top, you can safely deconstruct it later on to retrieve some of the resources. Note that archers / ballista weapons built on top of this height gain a massive range boost (about 16 blocks for archers), making them a very powerful inland defense.