Joker,
No, I haven't had any trouble getting the discount at all, in fact with all the construction mess Matt has been very flexible with prices.
I'd also like to add my kudos to the staff at the Amistad. Super-friendly, very willing to be of help. They're dealing with a difficult and messy work environment, but all seemed to be in friendly good humor about it. Wendy, Gabi and Flori treated me like a little brother...funny, since all are younger than I.

Alejandro the night guy is very intelligent and can be good company; don't let his brusque manner fool you. Rudi and Carlos at the front, along with "computer wiz" Marlene were able to get me what I needed when I needed it, including a small but significant 16.5% discount on top of the already generous 35% discount I received when I stayed in room 310 and paid cash.
I will say that depending on the room, noise is a bit of a problem. Even without the construction noise. Here's a funny thing: The closet in the actual penthouse has a window looking right down on the bed in 310!

Thankfully, the occupant didn't seem inclined to push aside his clothes for the sake of a peek...I hope.

He did comment one night that my chica and I got a little loud.
The Amistad is what it is, a hotel with almost a youth hostel feel. I really don't know (because I haven't yet stayed at the Castillo) if there is indeed a better option for its price range, particularly in barrio Amon. The Amistad is a block away from an excellent farmacia and two blocks from a hospital. It's located two blocks from the SL. Getting a taxi was never a problem, regardles of time of day.
I've gotten very, very nice rooms at the Amistad for relatively little money, the breakfast is good, the staff is first-rate,, the owner is a good guy and CRT sponsor/member.
I think that once the extensive remodeling has been completed that the Amistad may become my first choice for a hotel, particularly if the SL is booked. Which happens a lot.
