A large number of units with the pathfinding of well... a brick.
It's a great concept, and I could easily see this working as a low budget PC game for kids. Every aspect of it feels like Age of Empires, the medieval setting (well for the first campaign) and the resource gathering/base building is great.
However, it's flaws quickly show. To unlock bonus items, you must explore every inch of the map for blue bricks. This is fine, but the path finding is worse than lost magic, you have to move in straight lines or your army gets confused. It's not just slightly annoying, it completely ruins the game.
There's lots of other flaws, but I won't go into detail bashing em. For one, your units in the medieval campaign gather resources from trees. This is fine, and you have to build lumber yards etc to speed it up, and theres always the risk of depleting your supply. However, the hero unit can instantly create trees every 10 seconds or so, so all strategy from resource gathering is completly destroyed. It feels too much like cheating.
That said, the graphics are nice and there are 6 sizeable campaigns to play through. The cutscenes are also up to the lego standard. It's only a shame we didn't see some trademarked lego in here, such as Star Wars or Indy, they could have fit this game nicely.
Overall, only get this game if you can get it cheap, as the pathfinding ruins what is otherwise a great DS title.