Starcraft AI blog

Steamhammer’s ostensible Killer Feature or tournament surprise was burrowed zerglings to block the opponent from taking nearby bases. The lings are assigned to Watch squads to watch the bases, and burrow there as soon as burrow is researched. How did it work out? Do the Watch squads contribute to Steamhammer’s strength?

My conclusion: Yes. The cost is the expense of researching Burrowing (100/100) plus the assignment of up to 4 zerglings to watch enemy bases, taking them from other duties. It’s reasonably low. To pay that back, burrowed zerglings have to gain intelligence or delay enemy expansions. A few opponents, like Tscmoo, know what to do about burrowed zerglings and send detection and force. Even so, the intended expansion is delayed, and Tscmoo then starts the base immediately so that the zergling’s residual vision sees it and Steamhammer can react instantly. It’s already worth the cost. Other opponents treat a burrowed zergling the same as a spider mine, which is not always astute. XIMP sends a zealot to “trigger the mine” with no effect; the burrowed zergling continues to block the base until carriers fly over with an observer. It’s no better when terran scans and tries to kill the “mine” with an SCV. Some opponents don’t react and can’t deliberately expand at all; they are stuck until the zergling is removed by mischance. In many cases, I think Steamhammer would benefit by researching burrow earlier. There is virtually no effect against zerg, because by default the strategy boss researches burrow when Steamhammer has 3 bases, and that is late game in a ZvZ.

A couple issues limit effectiveness. First, the zergling unburrows when it knows that it is detected—and then it forgets and reburrows at once, going into a loop. The intention was to stand up and fight or flee when necessary, but there was a bug. Second, in an emergency when all other zerglings are killed, the last watching zergling unburrows and races home to join in the defense. But one zergling running in from afar barely affects the fight, and the enemy becomes free to expand without hindrance. After seeing games, I concluded that it will be more effective to leave the last watcher in place. Both issues are fixed in the development version. Overall, this is a low rate of errors compared to other new features I’ve made; care in testing paid off.

Against some opponents, the Watch squads do little good and represent mostly cost. The best example is Tyr protoss, which against Steamhammer plays one-base cannon defense into timing attack. When the attack comes, it includes an observer and the oncoming steamroller incidentally rolls up the zergling burrowed in the natural. Steamhammer pays the cost, and Tyr bypasses it with no effort. Blocking later bases sometimes helps, though. Another example is Iron, which likes to build turrets before it starts the command center. The burrowed zergling at most gets to see a turret or two. At some point I will put the Watch squads into the upcoming opponent model skill system and have Steamhammer collect data on how well they work. That will help Steamhammer research burrow at the right timing, as late as possible while effective against this opponent, and never if it is never effective.

I have more uses in mind for burrow. In particular, I want to use it to protect drones from some attacks. Reaver drops usually don’t include an observer, so when the reaver lands, the drones should poof out of sight before they can be targeted. When the reaver is picked up, the drones casually reappear and continue work. That will be funny to watch.