Spring RTS Engine

Open source TBS?
http://en.wikipedia.org/wiki/Advanced_S ... ic_Command
http://www.asc-hq.org/

I played Battle Isle: The Andosia War but got bored within an hour, mostly because, as the wiki notes,

The AI is relatively weak, relying on mass frontal assaults.After the player survives the first few turns and the AI's overwhelming numbers using protected artillery and terrain advantage, the computer is usually unable to mount a proper defense.

Advance Strategic Command looks TERRIBLE, downloading now but expectations are super low

all the turn based I'll ever need is probably going to come from WinSPWW2 aka Steel Panthers, trademark is truly a gent for dropping a link into one of the threads last week.

Battle Isle 2 is one of my favourite games :)

Downloaded but never tried ASC...

ASC is supposedly move advanced than Battle Isle for human vs human games, however I like BI2 more (BI3 is mostly the same, but doesn't show you what happens in your view distance during opponent's turn, which is wrong imo) for the campaign.

Also making a custom map with bime (aka Battle Isle 3 Editor, fan-made map/mission editor. Google for bime092w.zip to find it, at least that's the last version I know about) and setting yourself against 4 AIs every of which has numbers advantage over you, and then winning that battle, is fun.

AI is really dumb otherwise, it likes to spam fuel trucks and other unarmed units for example if given the opportunity, so designing a map to be challenging requires some experience :)

I must say I didnt like battle isle but back then I played it for the video scenes and 3d animations, so that doesnt say much about the game design.
I guess the turn based thing was too boring to me or jagged alliance or c&c were more intersting.