Flooring Tile - Tile on a concrete subfloor.

I have reviewed the postings but thought it might be best to post my specific situation and ask for assistance.

I have a small entry area, 4'x4'. I originally installed a plywood subfloor over top the concrete floor and then layed vinyl tile on top. I have decided to replace the tiles with ceramic tile. Here's a few questions:
1- Can I remove the plywood and lay the ceramic tiles directly on the concrete?
2- If not, should I remove the plywood and install backerboard and then the tiles on top?
3- If I can leave the plywood, should I screw them into the concrete because the plywood is currently only being held to the concrete with liquid nails. It seems to have been fine for two years but don't know if it will continue to hold.

Thanks :)

The plywood and adhesive need to go. Scrape the concrete smooth. High spots will cause the tiles to be lipped and uneven at the edges.

As long as there are no cracks in the concrete foundation in your entry/installation area, use a modified thinset, get muddy and plop tiles.

If there are cracks in the concrete, you may want to roll on a anti-fracture membrane, such as Red-gard.

I can confirm that there are no cracks in the concrete so I just need to clean off the adhesive that is remaining.
Do you recommend a specific modifed thinset?
and just to confirm, I do not need to lay down any type of backerboard or anything, just lay the tiles right on the concrete?
Thanks-

As long as the concrete is clean, smooth, and crack free, your good to go.

It is hard to beat Custom Building products: Flexbond. Home Depot carries it.

Thanks. I am going to try and surprise my husband with this one. :)