System-tracked structural health of the Nest Tower, announced in warnings as it collapses toward zero.
Warnings arrived in sequence as the Nest Tower took fire: integrity at 25% after Yarilo threw a building into it, then 10%, and finally 8% once Donut killed the last enemy tower shooting at them. With every mage either dead or trapped in The Ball of Flesh, the crawlers had no way to repair it, so the number only ever went down.
The countdown clock on whether the crawlers' last tower survives the siege.