Crystal Tower is supposed to be an easy 24 players dungeon. It is meant for players to gear up for Coil of Bahamut which is the hardest dungeon so far.

Right now, iLevel 70 is more or less the minimum to attempt Coil. You only need iLevel 70 and you obtain iLevel 90 gear (which will become iLevel 95), therefore a gap of 20 (soon 25 iLevel). This is not any different than Crystal Tower awarding iLevel 80 gear to iLevel 55 (a 25 iLevel difference, same as Coil).

As far as challenge goes, the true difficulty in 24 players dungeon is to organize said players. Difficulty-wise, each player is responsibly for 1/24th of the group success. On the other hand, in a 4 players dungeon, each player is responsible for 1/4th of the success or 6/24th compared to 24 players group. In a 4 players team, a dead weight in a challenging dungeon means automatic failure (unless the 3 others are over-geared for the dungeon). In a 24 players team, it takes 6 players for the same level of responsibility.