Are there day and night?
There is no day/night cycle in ToS.
I heard you can jump on objects. Is it possible to find new stuff or do random things with it? I'd like to find a hidden dungeon or items.
There are some things located where it can not be seen normally.
Is there a marriage system?
No, but if it's on high demand we might add it later.
What really is the 'costume'? Is it kind of a uniform? A collection? Just an option?
In ToS, 'costume' is for your character's appearance and it's independent from 'armors' that have defensive stats. It won't influence any of your characters stats. The costume of each class can be obtained from each class master NPCs once you unlock the class.
I've seen the time-reversing ability by a Chronomancer... Would it revive a dead ally? What about the items I've obtained earlier? How about the dead monsters?
First of all, it is on long cooldown so it can not be used that often. However, it will revive enemies and allies. Items will be kept. Think of a character in a SF movie doing the time-travel but still having objects in his pocket.
Thaumaturge, is a class that controls elements.
You may convert an enemy's trap to yours, or grow or shrink the size of the enemy. Some classes have abilities that depend on the size of the target object so it will influence the situation in combat. You can also grow a part of the body, so if you grow the hand/arm that's holding a weapon, it will give bonus to attack; The hand/arm with a shield, defense.
Archer, is a basic class for ranged combat classes.
The damage depends on the distance to the target. You may also switch stance to increase range, but you won't be able to move. Monsters attacked by strong/heavy arrow attacks sometimes get 'threaded' together. You can attack them like treating one target. It is a class that can attack while moving so it has upperhand in dealing boss monsters.
Pyromancer, is a class that uses fire.
Fire attacks do a lot of damage over time so it's favorable when fighting against a lot of enemies. You can use move keys while channeling 'Fire wall' to draw the shape of it. Fireball can be placed and moved by melee attacks so it can be used for several functions in party play.
Centurion, is a class that can lead other characters.
Players who joined the 'formation' will yield the control of their characters to the centurion. Each formation has its own unique passive. For example, 'Winged Formation' that's used to surround the enemy is efficient against boss monsters. 'Phalanx Formation' will transfer 'blocking stats' from rear side characters to the frontline so it is efficient in attacking a lot of enemies.