The dungeon will reset two hours after you leave.
Zelligar's Castle is a dungeon filled with golems and, at the heart, is Zelligar himself. Golems can spawn additional golems while you're fighting them. Zelligar has the coveted headband which is a goto piece of gear.
There are some steps to getting to Zelligar, which are available on a map here.
Zelligar is much harder than the golems and his well above his class. He will summon golems to assist him in combat. Zelligar is fearable and the golems are peaceable. Assuming the rooms immediately before Zelligar are clear, you can bounce.
Zelligar's headband is why Zelligar's Castle is a popular destination. It provides +int, +wis, damage mitigation, and HP and SP regen. Higher scaler levels provide larger benefits.
|? - ?||+1 int, 1/1 regen, stuns|
|? - ?||+2 int, 2/2 regen, stuns, mitigation|
|50? - 69||+3 int, 3/3 regen, stuns, mitigation|
|70 - 89||+4 int, 4/4 regen, stuns, mitigation|
|90 - 129||+5 int, +5 wis, 5/5 regen, stuns, mitigation|
|130+||+6 int, +6 wis, 6/6 regen, stuns, mitigation|
(Scaler information needs to be more accurate and Zelligar always hits hard!)