How do I check collisions? I've got player gravity down, but the character passes throughthe floor! What do I do to ensure it doesn't happen?

You can try code like "IF PLAYERY>203 THEN PLAYERY=203" for the bottom ground (assuming that the Y for the player is based off of pixels). You can set up the player collision with SPCOL 0,1 and set SPHITRCs up to detect the collisions between blocks.

That's what I ended up doing.