What level is recomended? - For the three lower abyss forts it looks like level 35 and up.
Whats the Objective -Well looking at the Sulfur one it says clear the 63 elites in under 15 mins. This doesn't seem possible also I think the loot - Goblet, statue etc can be traded in for AP points but not sure
Goblets, statues, etc are traded in for like 800AP, but only 3 drop I believe. Sulfur instance has a treasure box in it that has a 15 min timer, and takes about 11.5min to kill it. It is pretty much impossible unless you have a chanter with Word of Wind up.
Siel's West has six bridges you need to clear, and at the end of the bridges there's rooms with chests in them, I only did it once so I have no clue what you are supposed to do and how to do it the fastest. Once again it is a 15 min timer deal.
I have never done Siel's East. Someone try it out! It probably has a 15 min timer where you need to clear to a treasure chest or something of that nature.