I'd cast another vote for Playmobil here. Not only are Playmobil sets filled with lots of very cool little details (tinier pieces can always be put away till an older age if necessary), but they are very expandable should your ds end up getting really into it (I know you can buy all kinds of expansion parts to make the castle itself much bigger, as well as there being tons of accompanying castle/knight themed sets), AND they have great re-sale value (should he end up not being so into it after all). I know you mention that you don't think he is old enough yet, but my ds has been playing with his older sister's playmobil sets since he was 2-ish (maybe a hair past 2, but younger than 2.5) and now will be getting some of his own sets for Christmas. And, like I mentioned before, the extra-tiny-easily-lost bits and pieces can always be put away till he (and his younger sib) are older. If you are looking for play value (as in: fun to play with and has the likelihood of being played with for many many years to come), then I don't think you can go wrong with Playmobil.
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(I promise I am not a Playmobil spokes-person! LOL!)