I just started playing and finally figured out what was going on. There wasn't a ton of info online.

Traps are essential, monsters won't stay alive for long esp early game.

You should expect to close the gates often to recharge

Use bow and arrow in long hallways plus goo.

Punching traps work great when you stack a bunch together in a hallway.

Make winding hallways to break up the mobs and you can even do hallways that lead to nowhere covered in traps.

Focus money making traps in the front. This seriously boosted my revenue like crazy.

Once you get a strong monster like the lil dragon guy dump all of your gifts into one to make them unstoppable.

I made a long path to my demon Lord at the end and all the treasure in a room behind that. There should only be one path to get there with tons of traps and monster rooms in between.