I stayed 1 night in this small, friendly and really well-cared for hotel. The rooms are really special, colonial feel, rustic/antique house, good bed and matrass, tip-top clean and cared for. The owner, Tia Marta, is available and will give you advice about where to go and what to see (and also will leave you alone if you prefer). There is a lovely little pool for cooling down after Cartagena's heat and really romantic patio with beautiful flowers. Location is good, because easy to get a taxi and quick to get/get away, but also really in the old part of town, so best of both worlds. Highly recommended.
I was in Casa Marta for Christmas and New year's with my wife and kids. We had a wonderful time, Cartagena is a beautiful city, Marta is an amazing host. We had a great time!