I'd be honestly surprised if no one suggested this before as a defensive measure, but it's worth a shot:
Water.
It's simple and, as I'd imagine, easy to implement. With the dawn of CTF, buying water blocks at a couple points apiece would be great for defending your flag room, as water blocks in the doorway would force any attacker to TNT first (or attempt to get through, slowed).
Water.
It's simple and, as I'd imagine, easy to implement. With the dawn of CTF, buying water blocks at a couple points apiece would be great for defending your flag room, as water blocks in the doorway would force any attacker to TNT first (or attempt to get through, slowed).