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!)
Scaler: 45: Zelligar the Novice Golem Master ~ 510k 58: Golem : ~200k Zelligar the Apprentice Golem Master ~2MIL 63: Golem: 170k - 190k 64: Zelligar the Apprentice Golem Master 3.3 Mil 65: Golems: 220k - 250k Zelligar the Apprentice Golem Master ~ 3 Mil 70: Straw Golem (341405) Zelligar the Learned Golem Master (4047380) 75: Golem : 441145 Zelligar the Learned Golem Master <4MIL 80: Golems avg 500-600k Zelligar the Learned Golem Master (6307483) 81: Golems avg <500k 82: Golems avg 1.5m 90: Steel Golem (2161225) Zelligar the Superior Golem Master (22772482) 92: Mud Golem (2492145) Zelligar the Superior Golem Master (21,424,035) 100: Glass Golem : 4,059,135 Zelligar the Superior Golem Master : 28,147,866 130: Golem : 6.4 ~ 7.1 MIL Zelligar the Supreme Golem Master : 40 MIL