It's a submission for the LOWREZJAM, hence the super low resolution (64x64).
- What you call a "cross" is an arrow 3x5 pixels large.
- What you call a "box" is a stone for ballista.
- "Call" does exactly what is said in the description - decreases your turn count, so you need to stand the siege 1 turn less to win.
- To use an Attacker click it and then click an enemy unit. Attackers become available the next turn after they've been played.