Simple. Blocks that cost 50 coins each that cant be blown up. Only mined.
It would have a deep slate level mineing time. it would add a fun layer of stratagy onto defense building. Espically on the entrences to the flag room
I myself am a defense player, i build stair walls to keep the base safe, and make a single entrence and exit point thats out of the way for the enamy.
But a problem arises with tnt spammers who ruin a good few hundred gold worth of the most important part of ctf
Defense.
I already have to contend with an overly cheap arrow upgrade. Capture the flag isnt just about captureing the flag, so lets encourage more people to play defensively as its intended.
By makeing pooring us wall builders hard earned money into defenses, Feel worth it when people who know what there doing for once, spam tnt, our one weakness.