The Ancient Sentry behaves like the Silver and Purple varieties, but it has more HP, and fires slightly faster. Like the other Sentries, it tends to get stuck in corners, and can usually be taken out from a distance or from behind a barrier.
Once corrupted by an Address Bomb, it moves away from and fires away from the spot it was corrupted at. It no longer moves toward or fires at Trace, so it's mostly harmless.
Certain varieties will actually do nothing while corrupted. They will still flash, but won't act afraid like the other versions. You can identify these versions by the different colors and flash pattern when corrupted.