Re: DMM Games in General
Hard to bunch them all up into a single description.
Typically they're browser games, but a few require downloads. They released their first 3D MMO just a bit ago. For any of the popular DMM games, there are threads on ULMF for them with guides, as well as people talking about new events and such. Just search the DMM tag and you'll find a good list of games. Check out the ones with the most posts.
The "Best of the Best on DMM"/ones I have personal experience with are:
Aegis:
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Unique gameplay (Tower Defense of sorts) among DMM games and it's single player. RTS with sprite animations and constant events. Regularly the top game on adult DMM (When there isn't a new game people are flocking to briefly).
It's the longest DMM game I've ever played (Way over a year and I'm still fully active), the constant new content and updates and events are really what make it. You won't easily grow tired of it if you enjoy the first battles since they work very hard to keep it up to date for new and old players. They're the nicest game on DMM to/for free players I've ever seen. You can acquire about anything as a free player with time. Though there have been a lot more exclusive units as well lately.
H scenes are a bit vanilla but the art/cgs are almost all great quality. They've even been updating the sprites recently.
Lord of Walkure:
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Typical formula for RPG Card Battle games that has spawned dozens of clones on DMM. It doesn't do much extra over the massive sea of copy cats, but it is far ahead in updates and content because of its life time. Multiplayer is "Raid" like events where your friends help you beat down bosses.
I played this for quite a while before I got tired of it, problem I had with it was that you need to rely on other people to do everything eventually since the good cards are drops from raid bosses. If you don't have active/helpful friends, you won't do well in events. It's not as kind as Aegis to free players, you'll miss out on a lot of the best cards/girls and you'll FEEL it (Unlike Aegis). The way you upgrade cards means that if you don't acquire 4 copies of a girl, she may be utterly useless. It makes a lot of the event rewards seem terrible (They'll give you one or two copies of a card and make you roll premium gacha for the rest).
I haven't played it in a while so I don't know the state it's in, but it's still usually in the top 3-5, so I assume it's doing okay. Among the card battle games on DMM (And there's a lot of them) it's still probably the best. Just bring your wallet, it is not kind to free players
Tentacles Tactics:
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2D "Spawn minion" type RTS game. I found it really charming and it's pretty unique. The battles are fun to watch and the H scenes are pretty interesting/extreme if you're into that (Tentacles, of course). You actually directly control how most things play out via how you spawn your minions, and I love the tug of war feeling it has. It also has a surprising amount of depth strategy wise (If you avoid the broken units). I stopped playing it a while ago because it can become EXTREMELY grindy, and not in a fun way. Maybe it's a bit better now but there wasn't much content when I played. Good news is that it had a lot of potential, I wonder how it's doing now.
When I played there was a lot of trouble with unit balancing but it could be better now. Some units were must haves on any team and others were completely useless. Multiplayer is pvp but the balance on it was pretty silly when I last played (Rock Paper Scissors system) and was easily exploitable.
They also aren't super nice to free players as without an inventory expansion or two, you'll have a tough time keeping all the girls you want. So if you're the type who enjoys collecting girls, either be prepared to pay a bit on expansions or maybe avoid this. The default girl limit is harsher than most.
Pero Pero:
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Card collection game without much of any "battles". Multiplayer is typically "raid" events like LoW where you help other players attack bosses and such. There is a decent variety of event types, but they're all hella grindy. There's almost always something to do in it and it has voiced sex animations instead of the typical CG, so if you're just aching for H content it may be the way to go.
Downside is it's probably the most life consuming of the bunch (if you want to do decent in events). I stopped playing it because I found myself just waiting for my resources to refresh constantly (Like LoW). Maybe it's a personal preference, but those "multiplayer raid events" that Pero and LoW have bother me after a while. In most the events, you compete directly against other players for highscore rewards, so you know a free player has to sell their soul to have a chance at them since paying players can buy resource replenishes like nothing.
Flower Knight Girl:
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Although it's number 1 on the site right now, it's because it's rather new so people flock to it. I am enjoying it though even for it being an automated battle system type game (Also a lot of these on DMM). This and Aegis are currently my only active DMM games (Not counting the Closed Beta trial for Troy Game Princess/Troya Senki, the MMORPG). You set up teams of fighters and set them lose on a map. The battles are decently animated and pretty fun to watch for a while, but I'm not sure how long that will last. If you don't feel like doing much/giving much attention to a game, this is probably the AFK/Idle game I'd recommend right now.
Seems like you can acquire about everything so far as a free player if you're dedicated which is great, that's pretty rare in DMM games. It does have the dreaded multiplayer raid type system where you will probably eventually need friends to clear the battles, but the regular events seem great so far. Hopefully the raid events won't suck too badly. They only drop currency used to buy items right now (Rather than girls), so it shouldn't be too terrible. Also it's almost entirely RNG based when it comes to maps and fights, which can be pretty obnoxious if you like having control over battles.
Girl's Union
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City building/resource gathering game where you can attack other player's/npcs cities RTS summon unit style. A really unique game for DMM, not sure why it doesn't get much attention. Well last I played it, it was similar to Tentacles Tactics (Lacking content) so that could be why. I don't have much to say about it since I played it right when it was released and got bored because I reached the end so quickly, but it could be improved quite a bit by now. It certainly had potential.
Last I played it there were plenty of broken units though which made attacking or defending against certain people a stupid nightmare. You also don't get much play time out of it since there's a lot of downtime to rehire your units. If you don't want to spend too much time playing a game, it's not a bad choice.
Games I don't have any experience with but do well on the site:
Taimanin Asagi:
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If you're a fan, why not. It's a card battle game and I've kind just been avoiding them after LoW. It's regularly in top 5 or so on DMM, so it's gotta be doing something right. I certainly like the art.