Lara Croft's 2010 jaunt into isometric puzzling is free-to-play through Core Online, but only if you're okay with having play interrupted by ads. The service's help page indicates ads should generally appear every 20 minutes, as long as you watch a full minute of sponsored messages at each interruption. On our testing we were offered a selection of three ads, each with different running times. 1 minute and 14 seconds of a Mini Ninjas ad gave us 25 minutes playing time.
If you prefer raiding tombs without people intermittently trying to sell you stuff, you can remove ads from the full game for $9.99, or from individual missions for $0.99 each.