Xykon and Redcloak probably cleared out most of the worst defences, but more importantly: Dorukan's Gate was secure because it had an Epic Wizard sitting on it.

Dorukan's Gate was only compromised because he flew into a panic and went out to throw himself at Xykon. If he'd kept his head, he could have cast Limited Wish for a Death Ward and ruined Xykon's whole day. Hell, he could have kicked back and researched Dorukan's Epic Smite Lich or something.

The Gates tend to fail because of a flaw in their guardians. Dorukan's was that for all his intelligence, he couldn't think straight when emotionally provoked.