Touch controls

One finger on the ground — place a block and drag to draw a line. Turn once without lifting your finger to create a corner.
One finger on top of a block — start an upper level. Drag to draw a new row directly above it, with one possible turn.
One finger on the side of a block — arrows appear. Drag to extend the wall at the same height, or drag upward to build higher.
Two fingers — rotate and pinch to zoom.
Move View mode — drag with one finger to pan toward the hill's edges.

Materials

Stone — your main wall material. It is heavy and weak when overhanging, so widen the base before building high.
Wood — light and able to bridge gaps: floors, walkways and wall walks. Wood cannot stack directly on wood; place stone above it. It has 52 HP instead of stone's 130.
Iron — not a block, but a reinforcement. Tap an existing block to triple its HP.
Archer — a unit rather than a block. It has 75 HP and shoots enemies within 9 metres. Attackers will target it only when their climbing ability gives them a real path to its position. It also falls if its supporting block collapses. Surviving archers recover fully between waves.
Between waves, touch and drag an archer to redeploy it. Its cell is occupied, so you cannot build through it.

Special defences

Tap Stone or Wood again to choose a recipe.
Spike pit — costs 3 wood and must be placed on an empty ground cell. While hidden, enemies do not avoid it. The first enemy falls in; once revealed, the open pit forces the others to find another route.
Portcullis — costs 6 wood and 1 iron. Place it in a one-cell passage between two walls. Enemies may attack it instead of taking a long detour. Its control button raises or lowers every portcullis during a wave.
Battlement (merlon) — costs 2 stone and sits on a stone block. An archer on that cell fires farther and faster, and takes 40% less damage when attackers reach it.

Building

A wall that is too thin collapses under its own weight. Blocks shown in orange will fall.
Rubble creates a ramp, so clear dangerous ruins.
Trees are natural obstacles: nothing can be built on their cells, and attackers must path around them.
You cannot build during a wave. Undo only affects actions from the current preparation phase. Once a wave starts, use Demolish to change older construction.

Hold the keep

Goblins climb 1 block, climbers 2, and ladder orcs 4. Sappers undermine foundations; trolls smash through everything, slowly.

Mouse

Right-drag — rotate · Middle-drag — pan
Wheel — zoom · 1 2 3 4 — material · Ctrl+Z — undo