FormerlyHemlock
Hero
That sounds like a perfectly rational approach for the villain to take to me.
One issue with D&D generally is that, as you discovered, there isn't a good framework in place for a DM to manage actions which take out of the PCs' sight. On the one hand, there's generally no way for PCs themselves to know what is happening out of their visual radius (unless it is really loud). On the other hand, it's kind of un-fun for the DM and the players if the crucial decisions are all being made by the DM without player involvement, and the players only get to see the end result of "after ten minutes of relative quiet, the three invisible mind flayers who have been sneaking up on you for the past five minutes Mind Blast everybody, and then four wights charge in the door while ten hobgoblins pour in fire with bows." Since the players didn't participate at all in constructing the ambush (except for taking ten minutes) you run the risk of it feeling like unfair metagaming on the DM's part.
I like your approach of controlling the whole complex, leaving perimeter forces intact while interior forces mass to counterattack. Another approach that I've used is to simply tell the players (not PCs) that I'll be rolling randomly to determine how many enemies arrive on any given round, out of the total number of enemies in the area. (E.g. out of 24 umber hulks, if I don't know what part of the neogi ship they're in, I'll say that "1d4 umber hulks arrive per turn".) This keeps the focus on the PCs, including my mental focus as DM, and gives them a measure of predictability at the metagame level while still keeping the pressure up. It's inferior to the detailed approach you used, but it's cheaper mentally for me and so I've used it more often. If I had better tools for managing a whole dungeon at once I would use the detailed approach more often.
One issue with D&D generally is that, as you discovered, there isn't a good framework in place for a DM to manage actions which take out of the PCs' sight. On the one hand, there's generally no way for PCs themselves to know what is happening out of their visual radius (unless it is really loud). On the other hand, it's kind of un-fun for the DM and the players if the crucial decisions are all being made by the DM without player involvement, and the players only get to see the end result of "after ten minutes of relative quiet, the three invisible mind flayers who have been sneaking up on you for the past five minutes Mind Blast everybody, and then four wights charge in the door while ten hobgoblins pour in fire with bows." Since the players didn't participate at all in constructing the ambush (except for taking ten minutes) you run the risk of it feeling like unfair metagaming on the DM's part.
I like your approach of controlling the whole complex, leaving perimeter forces intact while interior forces mass to counterattack. Another approach that I've used is to simply tell the players (not PCs) that I'll be rolling randomly to determine how many enemies arrive on any given round, out of the total number of enemies in the area. (E.g. out of 24 umber hulks, if I don't know what part of the neogi ship they're in, I'll say that "1d4 umber hulks arrive per turn".) This keeps the focus on the PCs, including my mental focus as DM, and gives them a measure of predictability at the metagame level while still keeping the pressure up. It's inferior to the detailed approach you used, but it's cheaper mentally for me and so I've used it more often. If I had better tools for managing a whole dungeon at once I would use the detailed approach more often.