-- Don't use tiles that require too many movement points (MVP) just because they look good to you. Anything over 300 makes movement a lot harder, in other words annoying. Use them sparingly and only for strategic reasons, if for instance you want to set up an ambush and want to slow down an opponent's movement so that your NPC's can attack more effectively. Be extra careful with the 600 and 900 MVP tiles as they are practically impossible to cross for most players.
-- Be careful how you place your stairs. Placing stairs is not exactly rocket science but it comes close… If the stairs are not placed as they should be visitors will get the very annoying "Someone is blocking" message and won't be able to go any further.
So let's say you want to place stairs on floor 0, to go up to floor 1. On floor 0 you will need stairs going UP and on floor 1 stairs going DOWN (to floor 0). If on floor 1 you don't place stairs going down to floor 0, visitors will be stuck on floor 1 and you don't want that.
But the character that's going up (or down) needs a tile on which to land. While in your dungeon, at the top right of your screen, next to DIFFICULTY you will see 2 numbers in brackets, looking something like this: [5,0]. These are coordinates of every square and they are the same on every floor. So if for instance you choose to place your UP going stairs on square [5,0] on floor 0, you have to make sure that on square [5,0] on floor 1 there is a tile and that it's free, i.e. you haven't placed a character on that tile. If there is no tile on floor 1, the visitor can't go up the stairs and will get the "Someone is blocking" message. Same thing will happen if you have a tile but a character is standing on it. Super annoying.
At the DECORATION shop in Town you can find stairs that go UP and stairs that go DOWN.
-- Be careful how you place your pillars as characters cannot walk through them. So if you place a pillar in a narrow passage, the character won't be able to move past it.
-- If you have a door at the entrance of your dungeon don't set the lock to too high. You do want lower level characters to be able to enter…
-- Every time you upgrade your dungeon, check that you don't have loot penalty. More on this here: viewtopic.php?f=7&t=424
-- Keep investing in higher loot. There are two main reasons why someone would visit your dungeon, one is experience, the other is loot. Upgrade your loot level regularly.
-- Don't place too many items to loot, especially if your loot is low. It's better to place fewer items with higher loot than a gazillion little vases that yield only 5 gold each…
-- Make sure all your items are clickable. Sometimes, when you place items too close to each other one will not be clickable, which means your visitor won't be able to loot it.
-- Always run your dungeon yourself, from beginning to end, to check that everything is OK. It's better to do this before placing your defenders, as they will attack you. But if you have already placed them you can always deactivate them i.e. make them peaceful for a while.