Constant

Everything looks fine? Not really. In code below, we don't know what 1 means.

public void FireOnInvaders(Invader[] invaders)
{
    foreach (Invader invader in invaders)
    {
        if (invader.IsActive && _location.InRangeOf(invader.Location, 1))
        {
            invader.decreaseHealth(1);
            break;
        }
    }
}

We should make it more readable.

private const int _range = 1;
private const int _power = 1;

public void FireOnInvaders(Invader[] invaders)
{
    foreach (Invader invader in invaders)
    {
        if (invader.IsActive && _location.InRangeOf(invader.Location, _range))
        {
            invader.decreaseHealth(_power);
            break;
        }
    }
}