Level two introduces us to colonisation, and the limitations of where you can send your seedlings. A colonised asteroid adds to your explorable area, and larger asteroids provide a larger radius in which to travel. This leads to some interesting opportunities: you can cut off over-adventurous units, leaving them unable to move. You should also be careful using a giant asteroid's range, as it could easily be a one-way journey. Finding bottlenecks and using this range to your advantage is one of the more interesting parts of the game, and it quickly becomes second nature.
Level three brings on the Greys - a mindless, angry threat, whose aggression upsets Mother Tree, your narrator. "They are mad with violence and anger. Why do they fight us?" Her answer comes in level four, where other seedling empires are ushered in. Upon meeting them, your seedlings instantly attack. Who knew that seeds are basically aggressive pricks? It also shows the limits of your interaction: all you can do with seeds is tell them where to go, and to build trees.
Attacking an enemy-controlled asteroid is a matter of overwhelming it with seeds, destroying one of their trees, and barrelling down its roots to infect the core. The lack of control is frustrating: you can't focus your attack on a particular tree, so your seedlings might destroy a tree with a flower - something you'd obviously prefer to capture. You inherit the surviving trees on the asteroid, but the destroyed tree is always replaced by a seed-producing Dyson. This means you have little say in the loadout of your asteroids, as you can't destroy and rebuild, either. Still, it's a spectacle - once the tree is destroyed, and the path to the core opened, your seedlings gain a sense of urgent purpose - hurling themselves at the impassive asteroid like the over-excited gametes they are. The asteroid only accepts one seed every so often, so a haze of rejected seedlings bounce from the surface. It's one of the most comical and charming uses of swarm AI I've seen.
So far, still so very simple. The remaining elements are drip fed. Defence trees - the second and only other form of tree - are introduced in level five. Level six tells you how to select seeds with a certain quality - double-clicking on a asteroid lets you select from strong, fast, or energetic seeds, and move them individually. A strange note, though - seeds can possess all three qualities. If all your seeds are strong, and some of them fast, there's no explicit way to leave the fast ones behind. An option to right-click to exclude certain seeds would be useful - at the point when your army is attacking the core, you really want to send non-energetic seeds away, and there's no way to efficiently do so.
The last thing you'll learn about is flowers. Produced without any discernible pattern by mature trees, flowers can be plucked off and planted on any asteroid to dramatically enhance a single Dyson or Defence tree. They also cause trees to grow the occasional mine - a strong weapon that can be despatched to defend or attack asteroids, delivering tri-lasered death to enemy seedlings.
And that's it. As immersive and engrossing as it can briefly be, the simplicity of the gameplay hobbles the game. Take a mission in which you're given a novel win condition: protect the greys. At first, it's a welcome change. But because of the unchangably hostile seedlings, this simply involves retreating when the greys attack. It's not a satisfying level at all, suffering the whim of swarm AI.