I have 196 Dwellers (kept below 200 on purpose). Many of my Dwellers are maxed level and SPECIAL. All my rooms are fully upgraded (I have one elevator shaft straight down the middle a single maxed room off each side... 41 total rooms.)
To combat the DC attacks my third room is a kill box. The first 10 Dwellers that DC encounter are all maxed level with maxed SPECIAL and have the best gear.
To AVOID attacks, I removed my Radio room, and my last dweller was sent out 5 hours and 46 minutes ago...
Without the door opening... the DC attacked.