Moon gates are portals that randomly appear in towns. If you have an active internet connection, they lead to the houses of other Elona players who have used the uploader utility. The English version of the uploader is broken in standard Elona (though it has been fixed in Elona+), so all currently existing moon gates lead to the houses of Japanese players.
When a moon gate is used, the game will save and then you will be teleported to a usermap randomly chosen from a list. The moon gate is consumed in the process, so it won't be there when you return. However, if you instead use a rune while standing on a moon gate tile, it will let you choose the area you will be teleported to, and only the rune is consumed - the moon gate will still be there when you return. (Elona+ players can choose maps without using a rune, but the moon gate will still be lost this way.)
Once in a house, you cannot interact with any object, with the exception of doors, altars, statues of Creator and summoning crystals.
Be aware that some houses are dangerous, containing hostile monsters and NPCs.
The gates themselves can be grabbed from the towns they appear, but are usually extremely heavy and will leave most PCs lethally "Overweight!". A standard moon gate, at 5000s, easily outweighs even gravestones, tombs, and superheavy weapons like Frisia's Tail, and (unless with a sufficiently high enough Weight Lifting skill) can instantly kill you upon picking it up before you get the chance to drop it. Runes are far lighter. Gates and runes are effectively furniture - they may have material types that affect their weight, and quality modifiers such as "maniac" or "godly".
Examples[]
Safe area: Harmless NPC's walking in harmless house that is likely filled with artifacts and such. Mostly players strutting their stuff.
Dangerous area: Filled with nasty surprises, like lv65 monsters in a maze.
Town invasion: Some random town, (for example, Palmia) filled with monsters and possibly NPC's fighting them.
Door areas: Areas filled with tons of doors, and possibly some high-level monsters in the middle. Good for training your lockpicking.
Bubble area: One of the fastest ways to level yourself up if you're well enough equipped (DV 50+ or so and they won't hit you). You're going to get surrounded by these things fast, so the teleport is important in case you get surrounded by the tougher slime type (only drain blood kills these in my case, but shuriken can kill them as well if a bit long). Also, be careful so you won't get starved. Fire Breath can also kill many of them a turn with 30+ Constitution.
Note that leveling in this manner is not recommended - your level determines the level of the monsters you encounter, but not your stats and skill level (aside from the few bonus points and the occasional feat) - you will end up being vastly underpowered for your level.
Sand bag area: An area filled with hostile or non-hostile tied-up girls and monsters. The non-hostiles recover health automatically, so feel free to sadistically beat them up as much as you want.(Note:If you're lucky you might turn up in one of those areas where a tied-up bubble monsters grant wishes.)
Godly wrath area: An area filled with hostile mobs. Last time I was here, didn't see any nearby exits, and in ~5 turns or so a rain of light beams will bombard down and eradicate every single person inside.
Battle area: An area where an incredibly powerful mage kills hundreds of monsters a turn inside a room. A great source of loot. An odd variation has the monsters choking to death on mochi instead.
Exporting[]
Moon gate maps are automatically downloaded to the user folder as temp.eum (for uploader version 1.0 and 2.0 maps) or temp.pum (for uploader version 3.0/Elona+ maps). You can retain moon gate maps by copying the temp file elsewhere and renaming it, then copying it back to the user folder. This will allow you to visit the map whenever you want via the Show House.
Cheating[]
A Cheat Engine-based hack called Elovac turns all moon gate usermap-based items to be 'yours' (i.e. you are able to pick them up). This is a great way to acquire non-wishable and otherwise rare items, such various hearts, eternal force weapons and maxed out living weapons. Changes on usermaps are not saved, so the actual map file is not affected, but newly-acquired items will remain in your inventory when you return to your world.